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>
diff --git a/Makefile b/Makefile
index 92faed6..eea69b1 100644
--- a/Makefile
+++ b/Makefile
@@ -729,8 +729,9 @@
 endif
 ALL-$(CONFIG_REMAKE_ELF) += u-boot.elf
 
-# We can't do this yet due to the need for binary blobs
-# ALL-$(CONFIG_X86_RESET_VECTOR) += u-boot.rom
+ifneq ($(BUILD_ROM),)
+ALL-$(CONFIG_X86_RESET_VECTOR) += u-boot.rom
+endif
 
 # enable combined SPL/u-boot/dtb rules for tegra
 ifneq ($(CONFIG_TEGRA),)