This moves the multiboot header into its own include file, simplifying head.S and making it easier to include/exclude the multiboot header based on config options. BUG=b:242829490 TEST=Able to build and boot google/rex. Change-Id: I59a22dfe36044b4dd64a5b028a134be7a7d02a48 Signed-off-by: Subrata Banik <subratabanik@google.com> Reviewed-on: https://review.coreboot.org/c/coreboot/+/82533 Reviewed-by: Julius Werner <jwerner@chromium.org> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>