Make default-config an overlay

This allows the root image to remain a [mostly] pristine
image, and turns default liveCD configurations into an
overlay, rather than copying them into the root image.

Signed-off-by: Simo Leone <simo@archlinux.org>

hooks/archiso

@@ -58,6 +58,10 @@ run_hook ()
 
     export LOOP_NUM="1"
     addon_dir="${BOOT_MOUNT}/addons"
+
+    # always layer default configuration
+    _mnt_squashfs "default-config.sqfs"
+
     if [ -e "${addon_dir}/config" ]; then
         msg ":: Mounting addons"
         while read img mountpoint type; do