Armin Ranjbar

Hi,
 
And welcome to the forum.
Most of your question are answered in here: https://docs.armbian.com/

Not yet, out of (more or less) supported boards Zero may be a better choice if you don't need onboard wireless (which is total crap) and a display output
While it's possible to build an image for the PC2, it's still in an early WIP state and mostly server oriented
 
Edit: Or if you want something like Orange Pi PC with onboard storage, look at Orange Pi PC Plus

No. Only boards with eMMC, Orange Pi Zero with SPI flash (which is optional but can be soldered if it's missing) and Orange Pi PC2 (which always comes with SPI flash soldered) can boot without SD card and without using FEL mode.
Depending on your project considerations you may have following options:
Using the cheapest possible SD cards only to store the u-boot Buying SPI flash on a breakout board and some wires to connect it like this (no soldering required, but you may want to find a way to attach it to the main board for a finished product) Choosing another H3 board if it's not too late

Well, as I said, you need some kind of permanent storage (one of listed there in theory, out of those one of supported by the HW (which leaves only external SPI flash and SD card). Additional u-boot environment tweaks can be done at compilation time with some patches if you need it.

Well, for a full PXE setup where do you want to load the u-boot from?
For example if you load only the u-boot with FEL without giving it a boot script, it will fall back to the TFTP/PXE boot, so if you can supply it with a correct image, it should work. You'll need to find more info for the "pxe boot" command and what image format does it need.
Alternatively you can change the u-boot environment to use TFTP without DHCP/PXE setup.

Hello armbian community
 
I'm also interested in this topic. So far I found following links describing FEL boot:
 
https://docs.armbian.com/Developer-Guide_FEL-boot/
http://linux-sunxi.org/FEL/USBBoot
 
I managed to load kernel, initrd and dtb to RAM (all files from /boot of armbian image):
 
sunxi-fel uboot u-boot-sunxi-with-spl.bin write 0x43300000 uInitrd write 0x42000000 zImage write 0x43000000 sun8i-h3-orangepi-pc.dtb
 
And start the kernel:
 
bootz ${kernel_addr_r} ${ramdisk_addr_r} ${fdt_addr_r}
 
Right now there are no kernel parameters so it hangs in initramfs.
 
 
Anybody who managed to NFS boot OPi PC without SD-card?

Full PXE boot and FEL/NFS boot are slightly different things.
 
FEL/NFS boot works fine for sunxi boards, it is used as an option for the quick testing of the WIP images during the development.
 
Please check this boot script for the additional kernel parameters, and this script for a working FEL/NFS boot implementation on top of our build system (uses this boot script template)
 
TL;DR you have to pass at least "ip=dhcp root=/dev/nfs nfsroot=<ip_or_hostname>:<path>" to the kernel command line