x86: Build a .rom file which can be flashed to an x86 machine

On x86 machines U-Boot needs to be added to a large ROM image which is
then flashed onto the target board. The ROM has a particular format so it
makes sense for U-Boot to build this image automatically. Unfortunately
it relies on binary blobs so we cannot require this for the default
build as yet.

Create a u-boot.rom output file for this purpose.

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