Create fs-overlay, post-build

Setup buildroot filesystem overlay and move our custom scripts/configs
into it.  Setup a post-build script and have it delete files generated
by buildroot that aren't in RescueOS v1.1.

This gets us closer to a buildroot configuration that will automatically
generate RescueOS images.  For now, we're using RescueOS v1.1 as a
reference.

fs-overlay/sbin/stfu

@@ -0,0 +1,2 @@
+#!/bin/sh
+killall battery-watchdog