Add U-Boot x86_64 payload

Currently seems to stall when booted from the GRUB
payload, but works when booted from the SeaBIOS menu.

I also tested it as a standalone payload and it seems
to boot. Will test on hardware next, and start adding
it to more mainboards.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/grub/default/config/payload

@@ -216,10 +216,10 @@ menuentry 'Load test configuration (grubtest.cfg) inside of CBFS  [t]' --hotkey=
 	fi
 }
 fi
-if [ -f (cbfsdisk)/u-boot ]; then
+if [ -f (cbfsdisk)/img/u-boot ]; then
 menuentry 'Load U-Boot (payload) [b]' --hotkey='u' {
 	set root='cbfsdisk'
-	chainloader /u-boot
+	chainloader /img/u-boot
 }
 fi
 if [ -f (cbfsdisk)/seabios.elf ]; then