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>

Kconfig

@@ -288,8 +288,11 @@ config ROM_NEEDS_BLOBS
 	depends on HAS_ROM
 	help
 	  Enable this if building the u-boot.rom target needs binary blobs, and
-	  so cannot be done normally. In this case, pass BUILD_ROM=1 to make
-	  to tell U-Boot to build the ROM.
+	  so cannot be done normally. In this case, U-Boot will only build the
+	  ROM if the required blobs exist. If not, you will see an warning like:
+
+	    Image 'main-section' is missing external blobs and is non-functional:
+	      intel-descriptor intel-me intel-refcode intel-vga intel-mrc
 
 config BUILD_ROM
 	bool "Build U-Boot as BIOS replacement"