x86: coreboot: Modify u-boot code to allow building coreboot payload

This prevents the preprocessor from complaining when processing
variadic macros

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Graeme Russ <graeme.russ@gmail.com>