Add documentation for custom firmware.

[Kaloyan501]

Is your feature request related to a problem? Please describe.
I cannot flash a custom firmware for the life of me. I want to boot Alpine trough U-boot. My PC has an hdd and the flasher, so im flashing to /dev/vdd. I have put in the unlimited cpu, 4x16MB RAM and 3 cards. I have tried the /mnt/builtin/firmware_flasher with a blank file + opensbi enabled, blinking cursor. Tried it with opensbi + u-boot.bin and uboot.elf from the debian repo, blinking cursor. Tried to dd fw_jump.bin to the chip, blinking cursor. Am I doing something wrong?

Describe the solution you'd like
A detailed documentation of how to flash a custom firmware.

Describe alternatives you've considered
None

[dhouck]

The OpenSBI file included in the mod seems broken to me. It works when I compile my own OpenSBI using the upstream repo and the low-memory patch that Minux uses, but that requires setting up a cross-compiler.