-fix install issue

install_minimal/airtime-install

@@ -62,10 +62,11 @@ python ${SCRIPTPATH}/../python_apps/create-pypo-user.py
 
 set +e
 php ${SCRIPTPATH}/include/airtime-install.php $@
-if [ "$?" -eq "2" ]; then
+$result=$?
+if [ "$result" -eq "2" ]; then
     #We've just finished an upgrade, so let's exit
     exit 0
-elif [ "$?" != "0" ]; then
+elif [ "$result" -ne "0" ]; then
     #There was an error, exit with error code.
     exit 1
 fi