ROCKPro64 onboard boot flash instructions

[Tobias Nygren <tnn%NetBSD.org@localhost>]

Hi,

I added the bits to allow us to install U-Boot to the onboard boot
flash. Here are instructions to migrate away from SD boot if you'd like
to give it a try.

1) Update to latest GENERIC64 kernel and rockpro64 dtb. It will give
you /dev/spiflash0.

rkspi0 at simplebus0: SPI
rkspi0: interrupting on GICv3 irq 85
spi0 at rkspi0: SPI bus
m25p0 at spi0 slave 0
m25p0: JEDEC ID mfgid:0xC8, devid:0x4018
spiflash0 at m25p0: GigaDevice 25Q127CSIG SPI flash
spiflash0: 16384 KB, 256 sectors of 64 KB each

2) Write U-boot to flash. Get rkspi_loader.img from
pkgsrc/sysutils/u-boot-rockpro64. (not rksd_loader!)

dd if=/dev/zero of=flash.bin bs=1m count=16 # pad to 128Mbit
dd if=rkspi_loader.img of=flash.bin conv=notrunc
dd if=flash.bin of=/dev/spiflash0 bs=64k 

3) Verify flash is correct:
openssl sha1 flash.bin /dev/spiflash0

4) Migrate MSDOS partition and root filesystem from SD card onto an
USB disk (or maybe SATA/NVMe? I didn't try yet. It should work if the
device has U-Boot support.)

5) Power cycle and remove SD card, it should say:
U-Boot SPL board init
U-Boot SPL 2017.09 (Aug 13 2019 - 23:11:14)
booted from SPI flash

Recovery note:

If you mess up the flash, place a jumper between PI-2 bus pin #23 and
pin #25 to force boot from SD card again.

Have fun,
-Tobias