x86: Update SPL for coreboot

At present SPL only works on bare-metal builds. With a few tweaks it can
be used for coreboot also.

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