bgpd: support for Ethernet Segments and Type-1/EAD routes

bgpd: support for Ethernet Segments and Type-1/EAD routes

This is the base patch that brings in support for Type-1 routes.
It includes support for -
- Ethernet Segment (ES) management
- EAD route handling
- MAC-IP (Type-2) routes with a non-zero ESI i.e. Aliasing for
  active-active multihoming
- Initial infra for consistency checking. Consistency checking
  is a fundamental feature for active-active solutions like MLAG.
  We will try to levarage the info in the EAD-ES/EAD-EVI routes to
  detect inconsitencies in access config across VTEPs attached to
  the same Ethernet Segment.

Functionality Overview -
========================
1. Ethernet segments are created in zebra and associated with
access VLANs. zebra sends that info as ES and ES-EVI objects to BGP.
2. BGP advertises EAD-ES and EAD-EVI routes for the locally attached
ethernet segments.
3. Similarly BGP processes EAD-ES and EAD-EVI routes from peers
and translates them into ES-VTEP objects which are then sent to zebra
as remote ESs.
4. Each ES in zebra is associated with a list of active VTEPs which
is then translated into a L2-NHG (nexthop group). This is the ES
"Alias" entry
5. MAC-IP routes with a non-zero ESI use the alias entry created in
(4.) to forward traffic i.e. a MAC-ECMP is done to these remote-ES
destinations.

EAD route management (route table and key) -
============================================
1. Local EAD-ES routes
a. route-table: per-ES route-table
key: {RD=ES-RD, ESI, ET=0xffffffff, VTEP-IP)
b. route-table: per-VNI route-table
Not added
c. route-table: global route-table
key: {RD=ES-RD, ESI, ET=0xffffffff)

2. Remote EAD-ES routes
a. route-table: per-ES route-table
Not added
b. route-table: per-VNI route-table
key: {RD=ES-RD, ESI, ET=0xffffffff, VTEP-IP)
c. route-table: global route-table
key: {RD=ES-RD, ESI, ET=0xffffffff)

3. Local EAD-EVI routes
a. route-table: per-ES route-table
Not added
b. route-table: per-VNI route-table
key: {RD=0, ESI, ET=0, VTEP-IP)
c. route-table: global route-table
key: {RD=L2-VNI-RD, ESI, ET=0)

4. Remote EAD-EVI routes
a. route-table: per-ES route-table
Not added
b. route-table: per-VNI route-table
key: {RD=0, ESI, ET=0, VTEP-IP)
c. route-table: global route-table
key: {RD=L2-VNI-RD, ESI, ET=0)

Please refer to bgp_evpn_mh.h for info on how the data-structures are
organized.

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