board/qemu_x86: don't enable u-boot

it's a bit buggy when building. disable for now.

will re-visit later.

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

resources/coreboot/qemu_x86_12mb/board.cfg

@@ -6,6 +6,5 @@ payload_grub_withseabios="y"
 payload_seabios="y"
 payload_memtest="y"
 grub_scan_disk="ahci"
-payload_uboot="y"
 blobs_required="n"
 microcode_required="n"