configs/*/airootfs/etc/systemd/system/systemd-networkd-wait-online.service.d/w...

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.

configs/baseline/airootfs/etc/systemd/system/systemd-networkd-wait-online.service.d/wait-for-only-one-interface.conf

 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
+# Allow systemd-networkd-wait-online to succeed with one interface, otherwise, if multiple network interfaces exist,
+# network-online.target gets needlessly delayed.
+# See https://wiki.archlinux.org/title/systemd-networkd#systemd-networkd-wait-online
 [Service]
+ExecStart=
 ExecStart=/usr/lib/systemd/systemd-networkd-wait-online --any

configs/releng/airootfs/etc/systemd/system/systemd-networkd-wait-online.service.d/wait-for-only-one-interface.conf

 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
+# Allow systemd-networkd-wait-online to succeed with one interface, otherwise, if multiple network interfaces exist,
+# network-online.target gets needlessly delayed.
+# See https://wiki.archlinux.org/title/systemd-networkd#systemd-networkd-wait-online
 [Service]
 ExecStart=
 ExecStart=/usr/lib/systemd/systemd-networkd-wait-online --any