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>

Makefile

@@ -1312,7 +1312,8 @@ quiet_cmd_binman = BINMAN  $@
 cmd_binman = $(srctree)/tools/binman/binman $(if $(BINMAN_DEBUG),-D) \
                 --toolpath $(objtree)/tools \
 		$(if $(BINMAN_VERBOSE),-v$(BINMAN_VERBOSE)) \
-		build -u -d u-boot.dtb -O . -m --allow-missing \
+		build -u -d u-boot.dtb -O . \
+		$(if $(BUILD_ROM),,-m --allow-missing) \
 		-I . -I $(srctree) -I $(srctree)/board/$(BOARDDIR) \
 		$(BINMAN_$(@F))