I added a basic page, documenting the U-Boot x86 payload that was recently added to Libreboot's coreboot setup:
https://libreboot.org/docs/uboot/uboot-x86.html
More info to be written at a later date, but this should be enough to get started.
Now I just need to update the build system docs (just a few tweaks) and finish the release announcement.
Libreboot release is imminent!
Reminder: U-Boot provides a basic UEFI implementation, and this is what it's used for in Libreboot, to provide UEFI Linux/BSD configuration.