bgpd: Refine check for which routes can be exported into VPN

bgpd: Refine check for which routes can be exported into VPN

A non-imported route or a non-VPN imported route is a candidate to be
exported into the VPN routing table for leaking to other BGP instances
or advertisement into BGP/MPLS VPN. The former is a local or learnt
IPv4 or IPv6 route. The latter is an IPv4 or IPv6 route that is based
on a received EVPN type-2 or type-5 route.

Implement a function to specify if a route can be exported into VPN
and use in the appropriate places.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>