zebra: support for adding L2NHG and mac-ecmp in the linux kernel

zebra: support for adding L2NHG and mac-ecmp in the linux kernel

Multihoming support requires a new dataplane feature, MAC-ECMP, to
bridge traffic to remote ESs that are attached to more than one
active VTEP.

As a part of this support indirection has also been added via
L2-NHGs. Using a nexthop group allows for fast failover
of MAC entries when an access port attached to a remote-ES goes
down i.e. instead of updating many MAC entries this becomes a
single NHG update to the dataplane.

Note: Some of the code here needs to be reworked to the new
dataplane model.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>