Update BIRD configuration.

If BIRD doesn't have the default route in its routing table it can get astonishingly confused about what routes are reachable. This change ensures that, rather than filtering the default route out when its imported from the kernel, we filter it out when exporting the route via BGP.

This ensures that BIRD can make the correct decisions about routing, without confusing every other node.

*Note: There's a follow-on problem, which is that if the device protocol doesn't correctly spot our network devices BIRD can still get confused about where to route to. That's a problem for another day.*