Some improvements to release tools

Change-Id: I651866d746a1e661fcefe0b28ef1e8e57cd502e9
diff --git a/tools/build/onos-prepare-release b/tools/build/onos-prepare-release
index 7d8f0db..ccf9152 100755
--- a/tools/build/onos-prepare-release
+++ b/tools/build/onos-prepare-release
@@ -26,7 +26,8 @@
 git checkout $BRANCH
 
 # Check existance of version
-git tag -l | grep -q $ONOS_VERSION && { echo "ERROR: Version already exists"; exit -1; }
+git tag -l | grep -q "$ONOS_VERSION\$" &&
+  { echo "ERROR: Version already exists"; exit -1; }
 
 export ONOS_ROOT=$DIR
 . $ONOS_ROOT/tools/build/envDefaults