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.

post-build.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+set -e
+
+target="$1"
+
+delfiles=" \
+  $target/etc/init.d/rcK \
+  $target/etc/init.d/S20urandom \
+  $target/etc/init.d/S40network \
+  $target/etc/securetty"
+for file in $delfiles ; do 
+  [ -e "$file" ] && rm "$file" || true
+done