x86: cros: Check ROM exists before building vboot

All the x86 devicetree files are built at once, whichever board is
actually being built. If coreboot is the target build, CONFIG_ROM_SIZE
is not defined and samus cannot build Chromium OS verified boot. Add
this condition to avoid errors about CONFIG_ROM_SIZE being missing.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>