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.

buildrootconfig

@@ -11,6 +11,8 @@ BR2_TARGET_GENERIC_HOSTNAME="rescueos"
 BR2_TARGET_GENERIC_ISSUE="Welcome to RescueOS"
 BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_DEVTMPFS=y
 # BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW is not set
+BR2_ROOTFS_OVERLAY="../N900_RescueOS/fs-overlay"
+BR2_ROOTFS_POST_BUILD_SCRIPT="../N900_RescueOS/post-build.sh"
 BR2_PACKAGE_BUSYBOX_CONFIG="../N900_RescueOS/busyboxconfig"
 BR2_PACKAGE_BUSYBOX_SHOW_OTHERS=y
 BR2_PACKAGE_TAR=y