linux_appimage: canonicalize release_dir argument

When running the script and providing a local path as an 
argument for the release directory, the rsync command would fail, because rsync 
changes the directory and the relative path no longer applies.

deploy/create_linux_appimage.sh

@@ -22,7 +22,7 @@ if [ ! -f ${QGC_SRC}/qgroundcontrol.pro ]; then
   exit 1
 fi
 
-QGC_RELEASE_DIR=$2
+QGC_RELEASE_DIR=$(readlink -f $2)
 if [ ! -f ${QGC_RELEASE_DIR}/${QGC_CUSTOM_BINARY_NAME} ]; then
   echo "please specify path to ${QGC_CUSTOM_BINARY_NAME} release as the 2nd argument"
   exit 1