x86: Change coreboot default build configuration to QEMU

QEMU is much easier for us test booting U-Boot as a coreboot payload
than having a real board like chromebook_link.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>