Only start GUI services if a screen is attached.

Support only for rpi's at this moment. Will figure out x86_64 when we get there.

buildroot-external/board/raspberrypi/rootfs-overlay/usr/sbin/screencheck

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+status="$(sudo tvservice -n 2>&1)"
+
+if [ "$status" == "[E] No device present" ];
+then
+  exit 1
+else
+  exit 0
+fi