ethernet: T6306: add support for EVPN MH uplink/core tracking
When all the underlay links go down the PE no longer has access to the VxLAN
+overlay.
To prevent blackholing of traffic the server/ES links are protodowned on the PE.
A link can be setup for uplink tracking via the following configuration:
set interfaces ethernet eth0 evpn uplink
(cherry picked from commit 5565f27d15c5e7378e94aae8db8a894a12e25d7b)