x86: Drop CONFIG_BUILD_ROM and repurpose BUILD_ROM

x86: Drop CONFIG_BUILD_ROM and repurpose BUILD_ROM

This Kconfig is not needed anymore since U-Boot will build the ROM if the
required binary blobs exist.

The BUILD_ROM environment variable used to request that the ROM be built.
Now this always happens if the required binary blobs are available. Update
it to mean that U-Boot should fail if the ROM cannot be built. This
behaviour should be compatible with how it used to work.

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