x86: Add an option to enabling building a ROM file

Rather than requiring the Makefile to be modified, provide a build option to
enable the ROM to be built.

We cannot do this by default since it requires binary blobs. Without these
the build will fail.

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