The standard image generated by U-Boot on x86 is u-boot.rom. Add a
separate image called image.bin for verified boot. This supports
verification in TPL of which SPL/U-Boot to start, then jumping to the
correct one, with SPL setting up the SDRAM and U-Boot proper providing
the user interface if needed.

Signed-off-by: Simon Glass <sjg@chromium.org>