bgpd: fix peer startup for labeled-unicast if linklocal address not found

bgpd: fix peer startup for labeled-unicast if linklocal address not found

Problem found in testing where ipv6 labeled-unicast prefixes were not received
on the peers if a "service networking restart" was issued.  Same problem would
happen with an ifdown/ifup on the link to the peer.  Found the problem to be
that peers would establish for labeled-unicast even if a link-local address was
not yet available on the interface toward the peer, causing updates to be sent
without a nexthop value. These were then rejected by the peer. Fix is to delay
peer establishment until after the link-local addresses are available.

Ticket: CM-16779
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed By: Donald Sharp <sharpd@cumulusnetworks.com>
Testing Done: Manual testing successful.  Bgp-smoke completed with no new failures