zebra: Ethernet segment management and support for MAC-ECMP

zebra: Ethernet segment management and support for MAC-ECMP

1. Local ethernet segments are configured in zebra by attaching a
local-es-id and sys-mac to a access interface -
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
!
interface hostbond1
 evpn mh es-id 1
 evpn mh es-sys-mac 00:00:00:00:01:11
!
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
This info is then sent to BGP and used for the generation of EAD-per-ES
routes.

2. Access VLANs associated with an (ES) access port are translated into
ES-EVI objects and sent to BGP. This is used by BGP for the
generation of EAD-EVI routes.

3. Remote ESs are imported by BGP and sent to zebra. A list of VTEPs
is maintained per-remote ES in zebra. This list is used for the creation
of the L2-NHG that is used for forwarding traffic.

4. MAC entries with a non-zero ESI destination use the L2-NHG associated
with the ESI for forwarding traffic over the VxLAN overlay.

Please see zebra_evpn_mh.h for the datastruct organization details.

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