Babel: Set onlink flag for IPv4 routes with unreachable next hop

Babel: Set onlink flag for IPv4 routes with unreachable next hop

If the next hop of a route is not a reachable address, the route should be
installed as onlink. This enables a configuration common in mesh networks
where the mesh interface is assigned a /32 and babel handles the routing by
installing onlink routes.

Thanks to Toke Hoiland-Jorgensen for the patch.