Minor code re-org for layer2 route-table

1. Removed plen_ from layer2 route-table. The plen can be derived from
   IP-Address family
2. Shuffled the code to have related code together
3. Removed un-used code
4. Added new API to add EVPN route for an interface

Change-Id: Ic93eaf3036328cb6133934e24bb26009deb5642f
Merge "Minor code re-org for layer2 route-table"