patch_stick.sh: add support for update_stick.sh

Newer versions of the LIP-Image may contain an
update_stick.sh script. Use this script instead of plain
git commands if possible.

Installer-USB-Stick/patch_stick.sh

@@ -4,6 +4,7 @@ set -e
 
 DEVICE=$(blkid -t LABEL=MULTIBOOT -odevice)
 MPOINT=/mnt
+UPDATE_SCRIPT="./scripts/update_stick.sh"
 
 if [ ! $EUID -eq 0 ]; then
     echo "You are not root..."
@@ -18,8 +19,15 @@ fi
 mount "$DEVICE" "$MPOINT"
 pushd "$MPOINT" > /dev/null
 
+if [ -e "$UPDATE_SCRIPT" ]; then
+    echo "Using $UPDATE_SCRIPT."
+    bash ./scripts/update_stick.sh
+else
+    echo "No $UPDATE_SCRIPT avaiable."
+    echo "Falling back to plain git commands."
     git checkout master
     git pull
+fi
 
 popd > /dev/null