I see those are old boards, but they should still work with the latest uBoot I think. Which version did you flash, the emmc or the sdhc (https://developer.solid-run.com/knowledge-base/i-mx6-u-boot/, https://images.solid-build.xyz/IMX6/U-Boot/), and what jumper config did you have/do you boot from SD card of from emmc?
We've successfully installed on both and for emmc we use this extlinux.conf:
cat /boot/extlinux/extlinux.conf
menu title Select the boot mode
TIMEOUT 30
DEFAULT default
LABEL default
LINUX ../zImage
INITRD ../uInitrd
FDT ../imx6dl-hummingboard2.dtb
APPEND root=/dev/mmcblk1p1 ro rootfstype=ext4 rootwait console=ttymxc0,115200 console=tty0 consoleblank=0 video=mxcfb0:dev=hdmi,1920x1080m60,if=RGB24,bpp=32 rd.dm=0 rd.luks=0 rd.lvm=0 raid=noautodetect pci=nomsi vt.global_cursor_default=1 loglevel=5 usb-storage.quirks=0x2537:0x1066:u,0x2537:0x1068:u fsck.repair=yes
LABEL armbian
LINUX ../zImage
INITRD ../uInitrd
FDT ../dtb/imx6dl-hummingboard2.dtb
APPEND root=/dev/mmcblk1p1 ro rootfstype=ext4 rootwait console=tty1 console=ttymxc0,115200 consoleblank=0 video=mxcfb0:dev=hdmi,1920x1080m60,if=RGB24,bpp=32 rd.dm=0 rd.luks=0 rd.lvm=0 raid=noautodetect pci=nomsi vt.global_cursor_default=0 loglevel=3 usb-storage.quirks=0x2537:0x1066:u,0x2537:0x1068:u fsck.repair=yes
LABEL recovery
LINUX ../zImage
INITRD ../uInitrd
FDTDIR ../dtb/
APPEND root=/dev/mmcblk1p1 ro rootfstype=auto rootwait init=/bin/bash
This is with an edited dtb for the extra serial port on J35
