Merge remote-tracking branch 'nl6720/RouteMetric'

* nl6720/RouteMetric:
  configs/*/airootfs/etc/systemd/system/systemd-networkd-wait-online.service.d/wait-for-only-one-interface.conf: document why the drop-in file exists
  configs/releng/airootfs/etc/systemd/network/: move RouteMetric= from [DHCPv6] to [IPv6AcceptRA]
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