video: Make white-on-black a video-device property

The CONFIG_WHITE_ON_BLACK setting is hard-coded at build-time. It is
useful to be able to control this when showing menus.

Create a property to hold this information, using the CONFIG as the
initial value.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/expo.c b/boot/expo.c
index 786f665..8ce645e 100644
--- a/boot/expo.c
+++ b/boot/expo.c
@@ -194,7 +194,7 @@
 	u32 colour;
 	int ret;
 
-	back = CONFIG_IS_ENABLED(SYS_WHITE_ON_BLACK) ? VID_BLACK : VID_WHITE;
+	back = vid_priv->white_on_black ? VID_BLACK : VID_WHITE;
 	colour = video_index_to_colour(vid_priv, back);
 	ret = video_fill(dev, colour);
 	if (ret)