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.
Since systemd 245, IPv6PrivacyExtensions can be set not just per
connection, but also globally for all connection with a configuration
file in /etc/systemd/network.conf.d/.
