pimd: definition of pim-evpn origination and termination devices

pimd: definition of pim-evpn origination and termination devices

Two devices have special significance to multicast VxLAN tunnels -
1. tunnel origination device -
This device is used as the source device to vxlan-encapsulate BUM traffic.
In the case of the default-vrf this is lo. And in the case of non-default
VRF this is vrf-net-device. This patchset is limited to default-VRF
underlay so all subsequent references of origination-dev are to lo. But it
is possible in the future to extend support to non-default VRFs.
Sample origination mroute on single-VTEP:
(27.0.0.7, 239.1.1.100)          Iif: lo         Oifs: uplink-1

In the case of MLAG we need to mroute traffic form the MLAG-peer so
we force the IIF to the ISL.
Sample origination mroute on MLAG-VTEP:
(36.0.0.9, 239.1.1.100)          Iif: peerlink-3.4094 Oifs: peerlink-3.4094 uplink-1

2. tunnel termination device -
This device is used in the OIL to indicate that packets matching the flow
must be vxlan terminated and overlay packets subsequently forward to the
tenants. A special device has been created for this purpose called ipmr-lo.
This is a simple dummy interface from the kernel perspective which has
special siginficance only to pimd which implicitly enabled pim on the
device and adds it to the termination mroutes.
Sample termination mroute:
(0.0.0.0, 239.1.1.100)           Iif: uplink-1   Oifs: uplink-1 ipmr-lo

PS: currently we default the termination device name to "ipmr-lo" but in
the future it is possible to provide a config command to set the
termination device.

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