x86: provide CONFIG_BUILD_ROM

Up to now we depended on an exported variable to build u-boot.rom.
We should be able to specify it in the configuration file, too.

With this patch this becomes possible using the new Kconfig option
CONFIG_BUILD_ROM.

This option depends on CONFIG_X86 and is selected in
qemu-x86_defconfig and qemu-x86_64_defconfig.

Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/README.x86 b/doc/README.x86
index 92f4128..772e8d2 100644
--- a/doc/README.x86
+++ b/doc/README.x86
@@ -80,11 +80,15 @@
 little bit tricky, as generally it requires several binary blobs which are not
 shipped in the U-Boot source tree. Due to this reason, the u-boot.rom build is
 not turned on by default in the U-Boot source tree. Firstly, you need turn it
-on by enabling the ROM build:
+on by enabling the ROM build either via an environment variable
 
-$ export BUILD_ROM=y
+    $ export BUILD_ROM=y
 
-This tells the Makefile to build u-boot.rom as a target.
+or via configuration
+
+    CONFIG_BUILD_ROM=y
+
+Both tell the Makefile to build u-boot.rom as a target.
 
 ---