u-boot: Add qemu_x86_12mb build

Add a U-Boot build for the qemu_x86_12mb board. The config is a copy of
the upstream "coreboot" defconfig, but with OF_EMBED=y.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

resources/coreboot/qemu_x86_12mb/board.cfg

@@ -6,4 +6,4 @@ payload_grub_withseabios="y"
 payload_seabios="y"
 payload_memtest="y"
 grub_scan_disk="ahci"
-#payload_uboot="y"
+payload_uboot="y"

resources/u-boot/qemu_x86_12mb/board.cfg

+ubtree="default"
+arch="x86_64"