Usage of additional loopback interface with ripv2 (and ripng)

[frederic-loui]

Additional loopback are currently not advertized with ripv2 and ripng.

[rwestphal]

@frederic-loui As we discussed in our debug call yesterday, VPP's loop0 interface isn't a real loopback interface, so any /32 address configured on it is marked as an unnumbered addresses and ignored by holo-rip. Fixing this problem involves not ignoring unnumbered addresses, which needs to be done carefully. I'll postpone fixing this for now.