x86: coreboot: Allow building an SPL image

Make a few adjustments to allow us to build an SPL image for coreboot.

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