fbdev: set default margin color to white

ev3dev: doing this so we don't waste space in the kernel command line.
This is the same as setting fbcon=margin:7

Author: dlech

drivers/video/fbdev/core/fbcon.c

@@ -166,7 +166,7 @@ static int info_idx = -1;
 /* console rotation */
 static int initial_rotation = -1;
 static int fbcon_has_sysfs;
-static int margin_color;
+static int margin_color = 7;
 
 static const struct consw fb_con;