For info, my MMC device includes 2 partitions. mmcblk2p1 includes /boot, and mmcblk2p2 contains linux folders.
As I performed the steps above by mounting mmcblk2p2 on /mnt/system, new linux image 5.10.43 was actually available in /mnt/system/boot.
However, I had to copy the files to mmcblk2p1 partition. Otherwise, my system starts on MMC without any change in linux version.
After generated the image, I apply the instructions below:
root@helios64:~# mkdir -p /mnt/boot
root@helios64:~# mount /dev/mmcblk1p2 /mnt/boot
root@helios64:~# cp -r /mnt/system/boot/* /mnt/boot/boot
root@helios64:~# reboot
Here are all the steps in case someone else has the problem and wants to repair his eMMC image...
1. download a previous Helios64 image from here...
https://wiki.kobol.io/download/
2. install the image on a microSD e.g. for Windows and macOS there is Etcher...
https://www.balena.io/etcher/
3a. boot your Helios64 with the microSD and Jumper 10, see here...
https://wiki.kobol.io/helios64/troubleshoot/#how-to-force-boot-from-microsd
3b. Alternative: from the serial console press a key on the keyboard while u-boot start. You will get the u-boot prompt.
From this prompt write and press enter. Helios64 will boot from SD... (thx @prahal)
run bootcmd_mmc1
4. if you are logged in as root user (normal users must put a sudo in front of the commands) execute the following commands...
root@helios64:~# mkdir -p /mnt/system
root@helios64:~# mount /dev/mmcblk2p1 /mnt/system
root@helios64:~# cd /mnt/system/
5. The contents of the /mnt/system directory should now be filled with the contents of your eMMC....
root@helios64:/mnt/system# ll
total 80
lrwxrwxrwx 1 root root 7 Aug 30 2020 bin -> usr/bin
drwxr-xr-x 3 root root 4096 Sep 1 03:01 boot
drwxr-xr-x 2 root root 4096 Oct 15 2020 dev
drwxr-xr-x 110 root root 12288 Sep 1 03:01 etc
drwxr-xr-x 2 root root 4096 Sep 22 2020 export
drwxr-xr-x 5 root root 4096 Jun 10 04:12 home
lrwxrwxrwx 1 root root 7 Aug 30 2020 lib -> usr/lib
drwx------ 5 root root 4096 Oct 5 2020 lost+found
drwxr-xr-x 4 root root 4096 Oct 15 2020 media
drwxr-xr-x 2 root root 4096 Feb 9 2021 mnt
drwxr-xr-x 4 root root 4096 Dec 2 2020 opt
dr-xr-xr-x 2 root root 4096 Oct 15 2020 proc
drwx------ 7 root root 4096 Aug 31 23:36 root
drwxr-xr-x 2 root root 4096 Oct 15 2020 run
lrwxrwxrwx 1 root root 8 Aug 30 2020 sbin -> usr/sbin
drwxrwxr-x 2 root root 4096 Oct 5 2020 selinux
drwxr-xr-x 6 root root 4096 Jun 10 03:18 srv
dr-xr-xr-x 2 root root 4096 Oct 15 2020 sys
lrwxrwxrwx 1 root root 42 Sep 1 03:01 thermal_zone0 -> /sys/devices/virtual/thermal/thermal_zone0
drwxrwxrwt 2 root root 4096 Oct 15 2020 tmp
drwxr-xr-x 12 root root 4096 Nov 27 2020 usr
drwxr-xr-x 14 root root 4096 Oct 6 2020 var
6. now download the 3 old packages...
root@helios64:/mnt/system# wget http://armbian.hosthatch.com/apt/pool/main/l/linux-5.10.43-rockchip64/linux-dtb-current-rockchip64_21.05.4_arm64.deb
root@helios64:/mnt/system# wget http://armbian.hosthatch.com/apt/pool/main/l/linux-5.10.43-rockchip64/linux-headers-current-rockchip64_21.05.4_arm64.deb
root@helios64:/mnt/system# wget http://armbian.hosthatch.com/apt/pool/main/l/linux-5.10.43-rockchip64/linux-image-current-rockchip64_21.05.4_arm64.deb
7. now changes the root directory...
root@helios64:/mnt/system# chroot /mnt/system
root@helios64:/# pwd
/
root@helios64:/# ll
total 50996
lrwxrwxrwx 1 root root 7 Aug 30 2020 bin -> usr/bin
drwxr-xr-x 3 root root 4096 Sep 1 14:56 boot
drwxr-xr-x 2 root root 4096 Sep 1 16:15 dev
drwxr-xr-x 110 root root 12288 Sep 1 03:01 etc
drwxr-xr-x 2 root root 4096 Sep 22 2020 export
drwxr-xr-x 5 root root 4096 Jun 10 04:12 home
lrwxrwxrwx 1 root root 7 Aug 30 2020 lib -> usr/lib
-rw-r--r-- 1 root root 314304 Jul 8 19:32 linux-dtb-current-rockchip64_21.05.4_arm64.deb
-rw-r--r-- 1 root root 11527696 Jul 8 19:32 linux-headers-current-rockchip64_21.05.4_arm64.deb
-rw-r--r-- 1 root root 40290884 Jul 8 19:33 linux-image-current-rockchip64_21.05.4_arm64.deb
drwx------ 5 root root 4096 Oct 5 2020 lost+found
drwxr-xr-x 4 root root 4096 Oct 15 2020 media
drwxr-xr-x 2 root root 4096 Feb 9 2021 mnt
drwxr-xr-x 4 root root 4096 Dec 2 2020 opt
dr-xr-xr-x 2 root root 4096 Oct 15 2020 proc
drwx------ 7 root root 4096 Aug 31 23:36 root
drwxr-xr-x 2 root root 4096 Oct 15 2020 run
lrwxrwxrwx 1 root root 8 Aug 30 2020 sbin -> usr/sbin
drwxrwxr-x 2 root root 4096 Oct 5 2020 selinux
drwxr-xr-x 6 root root 4096 Jun 10 03:18 srv
dr-xr-xr-x 2 root root 4096 Oct 15 2020 sys
lrwxrwxrwx 1 root root 42 Sep 1 03:01 thermal_zone0 -> /sys/devices/virtual/thermal/thermal_zone0
drwxrwxrwt 2 root root 4096 Oct 15 2020 tmp
drwxr-xr-x 12 root root 4096 Nov 27 2020 usr
drwxr-xr-x 14 root root 4096 Oct 6 2020 var
8. now installs the downloaded packages...
root@helios64:/# dpkg -i *.deb
dpkg: warning: downgrading linux-dtb-current-rockchip64 from 21.08.1 to 21.05.4
(Reading database ... 62558 files and directories currently installed.)
Preparing to unpack linux-dtb-current-rockchip64_21.05.4_arm64.deb ...
Unpacking linux-dtb-current-rockchip64 (21.05.4) over (21.08.1) ...
Selecting previously unselected package linux-headers-current-rockchip64.
Preparing to unpack linux-headers-current-rockchip64_21.05.4_arm64.deb ...
Unpacking linux-headers-current-rockchip64 (21.05.4) ...
dpkg: warning: downgrading linux-image-current-rockchip64 from 21.08.1 to 21.05.4
Preparing to unpack linux-image-current-rockchip64_21.05.4_arm64.deb ...
update-initramfs: Deleting /boot/initrd.img-5.10.60-rockchip64
Removing obsolete file uInitrd-5.10.60-rockchip64
stat: cannot stat '/proc/1/root/.': No such file or directory
Unpacking linux-image-current-rockchip64 (21.05.4) over (21.08.1) ...
Setting up linux-dtb-current-rockchip64 (21.05.4) ...
Setting up linux-headers-current-rockchip64 (21.05.4) ...
Compiling headers - please wait ...
grep: /proc/cpuinfo: No such file or directory
grep: /proc/cpuinfo: No such file or directory
Setting up linux-image-current-rockchip64 (21.05.4) ...
update-initramfs: Generating /boot/initrd.img-5.10.43-rockchip64
W: Couldn't identify type of root file system for fsck hook
update-initramfs: Converting to u-boot format
9. reset the root change with exit and restart your helios64 with reboot...
root@helios64:/# exit
exit
root@helios64:/mnt/system# reboot
10. after a reboot you should now boot from the eMMC again...
root@192.168.180.5's password:
_ _ _ _ __ _ _
| | | | ___| (_) ___ ___ / /_ | || |
| |_| |/ _ \ | |/ _ \/ __| '_ \| || |_
| _ | __/ | | (_) \__ \ (_) |__ _|
|_| |_|\___|_|_|\___/|___/\___/ |_|
Welcome to Armbian 21.08.1 Buster with Linux 5.10.43-rockchip64
No end-user support: built from trunk
System load: 74% Up time: 3 min
Memory usage: 34% of 3.77G IP: 172.18.0.1 172.19.0.1 172.20.0.1 172.17.0.1 192.168.180.5
CPU temp: 66°C Usage of /: 87% of 15G
[ 0 security updates available, 3 updates total: apt upgrade ]
Last check: 2021-09-01 16:38
[ General system configuration (beta): armbian-config ]
Last login: Wed Sep 1 16:26:23 2021
root@helios64:~#
root@helios64:~# uname -a
Linux helios64 5.10.43-rockchip64 #21.05.4 SMP PREEMPT Wed Jun 16 08:02:12 UTC 2021 aarch64 GNU/Linux
root@helios64:~#
11. if the eMMC was used you can see with mount...
root@helios64:~# mount | grep /dev/mmc
/dev/mmcblk2p1 on / type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)
/dev/mmcblk2p1 on /var/folder2ram/var/log type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)
/dev/mmcblk2p1 on /var/folder2ram/var/tmp type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)
/dev/mmcblk2p1 on /var/folder2ram/var/lib/openmediavault/rrd type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)
/dev/mmcblk2p1 on /var/folder2ram/var/spool type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)
/dev/mmcblk2p1 on /var/folder2ram/var/lib/rrdcached type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)
/dev/mmcblk2p1 on /var/folder2ram/var/lib/monit type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)
/dev/mmcblk2p1 on /var/folder2ram/var/cache/samba type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)