Fixing buck-built package for version roll.

Change-Id: I54fa47764df45c834b57be9cdc7dec28a30c25c3
diff --git a/tools/package/onos-prep-karaf b/tools/package/onos-prep-karaf
index 6564d5f..bbb6f0e 100755
--- a/tools/package/onos-prep-karaf
+++ b/tools/package/onos-prep-karaf
@@ -12,7 +12,7 @@
 #FIXME karaf version
 KARAF_VERSION="3.0.5"
 
-PREFIX="onos-1.6.0"
+PREFIX="onos-${ONOS_VERSION/-*/}"
 
 # Unroll the Apache Karaf bits, prune them and make ONOS top-level directories.
 tar xf $KARAF_TAR