fixing obf when $ONOS_TAR doesnt exist

Change-Id: I5612024438de251fa689cb3333d7efe76423e3f0

@@ -36,7 +36,7 @@ export ONOS_ZIP=$ONOS_STAGE.zip
 # If the BUCK-built bits are newer than the Maven-built bits, use the former.
 BUCK_TAR=$ONOS_ROOT/buck-out/gen/tools/package/onos-package/onos.tar.gz
 if [ -f $BUCK_TAR -a $BUCK_TAR -nt $ONOS_TAR ]; then
-    rm $ONOS_TAR >/dev/null; ln -s $BUCK_TAR $ONOS_TAR
+    rm -f $ONOS_TAR >/dev/null; ln -s $BUCK_TAR $ONOS_TAR
 fi
 
 # ONOS test bits (onos-test.tar.gz) staging environment