fix #5364: bgp|evpn: derivated router-id from mac address for ipv6 underlay

fix #5364: bgp|evpn: derivated router-id from mac address for ipv6 underlay

for ipv4, we use the iface ipv4 router-id as router-id need to 32bit.

That's doesn't work for pure ipv6 underlay network.

since https://www.rfc-editor.org/rfc/rfc6286, we can use any 32bit id,
it's just need to be unique in the ASN.

Simply use the last 4 bytes of iface mac address as unique id

changelog V2: add missing test

Signed-off-by: Alexandre Derumier <alexandre.derumier@groupe-cyllene.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>