Initial L2 LISP support

Initial L2 LISP support

This introduces support for layer 2 overlays with LISP. Similarly to L3,
all tenant packets to be encapsulated are captured by an interface, but
the mapping (layer binding) instead of being between an L3 VRF and a
LISP VNI, it is between and an L2 bridge domain and a VNI. At a high
level, this results in two important properties:
1) the source and destinations of all packets flooded in the
   bridge-domain are mapped via the LISP control plane and the replies are
   converted into data-plane tunnels tracked via a LISP specific
   source/dest L2 FIB
2) All packets reaching the interface and matching a source/dest L2 LISP
   FIB entry are L3 (IP4/6) encapsulated.

This is solely a unicast feature, therefore at this time ARPs are not
handled in any special way.

Change-Id: I0b7badcd7c6d5166db07d4acd2cc4ae7fba3e18e
Signed-off-by: Florin Coras <[email protected]>