WIP: Enable boot image generation
Now that the boot image can be generated (!87), do it for the devkit board.
This is WIP: since the requirements to merge this safely are not met yet.
Among other things, it depends on:
- having the download/flash scripts in Librem5/librem5-devkit-tools> be able to process both single-image and multi-image builds
- having the UUU scripts in Librem5/librem5-devkit-tools> be able to flash the U-boot and configure the eMMC to boot from the correct partition
- building a U-boot using the purism/librem5 board, with the env located in the boot partition
- other steps I forgot?