bgpd: fix export prefixes when rt extcomm set by route-map

bgpd: fix export prefixes when rt extcomm set by route-map

When exporting BGP prefixes, it is necessary to configure
the route target extended communities with the following
command:

> rt vpn export <RouteTarget>

But the customer may need to configure the route-target to
apply to bgp updates, solely based on a route-map criterium.
by using the below route-map configured like that:

> route-map vpn export <routemapname>

Fix this by allowing to export bgp updates based on the
presence of route-targets on either route-map or vpn
configured rt. the exportation process is stopped
if no route target is available in the ecommunity list.

Fixes: ddb5b4880ba8 ("bgpd: vpn-vrf route leaking")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>