configs/*/airootfs/etc/systemd/network/: add RequiredForOnline=routable

By default systemd-networkd-wait-online.service considers a network connection to be "online" when it has reached the "degraded" state (see networkctl(1) for the definitions). Since "degraded" does not ensure there's a routable address, let's change the connection's requirement to "routable" instead.

This gives a better chance that the network really is online when network-online.target is reached.