L2 Emulation

L2 Emulation is a feautre that is applied to L2 ports to 'extract'
IP packets from the L2 path and inject them into the L3 path (i.e.
into the appropriate ip[4|6]_input node).
L3 routes in the table_id for that interface should then be configured
as DVR routes, therefore the forwarded packet has the L2 header
preserved and togehter the L3 routed system behaves like an L2 bridge.

Change-Id: I8effd7e2f4c67ee277b73c7bc79aa3e5a3e34d03
Signed-off-by: Neale Ranns <nranns@cisco.com>