configs/*/airootfs/etc/systemd/system/systemd-networkd-wait-online.service.d/wait-for-only-one-interface.conf: document why the drop-in file exists

Related to #142.

Add missing `ExecStart=` to baseline's /etc/systemd/system/systemd-networkd-wait-online.service.d/wait-for-only-one-interface.conf.