networkd: ipv4ll - configure link-local address independently of DHCPv4

This changes the behavior when both DHCPv4 and IPv4LL are enabled. Before,
we would disable IPv4LL when we got a DHCPv4 lease and enable it if the
lease was lost.

Now we just always set up both, if both are enabled, but the DHCPv4
addresses and routes will always take precedence due to their metric
and scope.