Draft: U-Boot on SPI flash for veyron chromebooks

This packages u-boot.rom files for 4 boards supported in U-Boot (mickey, jerry, minnie, speedy). This file is about to be flashed to SPI flash and replace stock bootloader.

I tested it only on speedy, so I promote this only on speedy wiki page.

There is a problem with booting from eMMC, but it is easy to workaround and it will be fixed in pmbootstrap.

I've tried to package it so that if a new board is added to U-Boot, we only have to add another value to $_boards variable.

Config changes are suggested by Alper Nebi Yasak. He is going to work on these devices in upstream. Once it is upstreamed, we may consider to package this to Alpine, but I am not sure whether it is a good idea, because then we do not control version upgrades, and someone may flash a broken version. It is not possible to recover it without SPI programmer.

Link: https://wiki.postmarketos.org/wiki/ASUS_Chromebook_C201_(google-veyron-speedy)#U-Boot