U-Boot with direct NVMe boot support for eMMC/SPI Flash

[Arwen]

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Thanks!
SPI is a no-go for the same reasons Pine64 ships their U-Boot on eMMC: there is no safe way of flashing it and no easy way to recover.
Plus, I guess there's still work to do: it would be nice to have display and keyboard working in the pre-boot, before they're initialized by the kernel.

There is a program to write the SPI flash. How safe it is, I don't know. Perhaps I will play with it on my ROCKPro64, (same RK3399 & SPI). It has an easier way via header connector to disable the SPI flash if a U-Boot image does not work.

I did come up with a scheme to make using the SPI safer. Since it's 16MBytes, we can likely store 2 U-Boot images. The idea is to keep the working one, but add a new one for additional features. If the new one fails spectacularly, and prevents any other recovery, we power off. Then insert a special SD card, which the initial SPI image would check the SD card for a special signature. Like a 64 byte code at a specific location. Then fail back to the older, working U-Boot image. (Or a second 64 byte code that causes a boot to the SD card.)

Anyway, I will start a thread somewhere so we can pick it apart. And hopefully re-assemble it into something that would work for U-Boot on SPI flash.