video: add config option to skip framebuffer clear

The use-case is that the thing that loaded u-boot already put a splash
image on screen.  And we want to preserve that until grub boot menu
takes over.

Signed-off-by: Rob Clark <robdclark@gmail.com>
diff --git a/drivers/video/cfb_console.c b/drivers/video/cfb_console.c
index 6c5425c..74cc20d 100644
--- a/drivers/video/cfb_console.c
+++ b/drivers/video/cfb_console.c
@@ -2081,7 +2081,8 @@
 	}
 	eorx = fgx ^ bgx;
 
-	video_clear();
+	if (!CONFIG_IS_ENABLED(NO_FB_CLEAR))
+		video_clear();
 
 #ifdef CONFIG_VIDEO_LOGO
 	/* Plot the logo and get start point of console */