java_gen: small improvements to gen_table/tlv extensions

 - move bsntlv classes to dedicated package/factory
 - change type of of_bsn_tlv_vlan_vid to VlanVid
 - change type of tableId to new typesafe constant GenTableId
diff --git a/java_gen/templates/_imports.java b/java_gen/templates/_imports.java
index 498d2f9..0c95916 100644
--- a/java_gen/templates/_imports.java
+++ b/java_gen/templates/_imports.java
@@ -8,6 +8,7 @@
 import org.projectfloodlight.openflow.protocol.*;
 import org.projectfloodlight.openflow.protocol.action.*;
 import org.projectfloodlight.openflow.protocol.actionid.*;
+import org.projectfloodlight.openflow.protocol.bsntlv.*;
 import org.projectfloodlight.openflow.protocol.errormsg.*;
 import org.projectfloodlight.openflow.protocol.meterband.*;
 import org.projectfloodlight.openflow.protocol.instruction.*;