message: tools: cope with repo already downloaded
diff: @@ -13,6 +13,8 @@ MAVLINK_HEADERS_PATH=mavlink-headers
  if [ ! -d $MAVLINK_PATH ]
  then
  git clone --recursive $REPO $MAVLINK_PATH
  +else
  + (cd $MAVLINK_PATH && git fetch)
  fi
  (cd $MAVLINK_PATH && git checkout $TAG && git submodule update --init --recursive)
  