x86: Build vga video code only if CONFIG_VIDEO_VGA is defined

When running from coreboot we don't want this code, so make it
optional.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index c7aab18..0f5dfad 100644
--- a/README
+++ b/README
@@ -1409,6 +1409,13 @@
 		boot.  See the documentation file README.video for a
 		description of this variable.
 
+		CONFIG_VIDEO_VGA
+
+		Enable the VGA video / BIOS for x86. The alternative if you
+		are using coreboot is to use the coreboot frame buffer
+		driver.
+
+
 - Keyboard Support:
 		CONFIG_KEYBOARD