x86_64 U-Boot added to Libreboot, as a secondary payload, executable from the SeaBIOS or GRUB payload menus. Patches as follows:
https://browse.libreboot.org/lbmk.git/commit/?id=747b6514eaf00ac71af475daab98509bf8870b23
https://browse.libreboot.org/lbmk.git/commit/?id=f13819386bf35f081eecbacea965549b7df75e24
https://browse.libreboot.org/lbmk.git/commit/?id=637c0a1521a03e3f65de85dcc5ffd478b37a5360
Highly experimental. I added 32-bit U-Boot to Qemu the other week; U-Boot is now enabled on *most* mainboards (except ones with not enough flash). The 64-bit target has basic UEFI support.
This is using the work done by Simon Glass, Martin Roth and others in bringing U-Boot to coreboot on x86.