x86: provide CONFIG_BUILD_ROM

Up to now we depended on an exported variable to build u-boot.rom.
We should be able to specify it in the configuration file, too.

With this patch this becomes possible using the new Kconfig option
CONFIG_BUILD_ROM.

This option depends on CONFIG_X86 and is selected in
qemu-x86_defconfig and qemu-x86_64_defconfig.

Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>