add: TODO comment for future improvement

With systemd/systemd#9796 [1], included in systemd v240 and later [2],
we will be able to use `systemctl is-system-running --wait` to wait for
the system being online instead of polling systemd’s private socket,
which should be both more efficient and more stable.

[1]: https://github.com/systemd/systemd/pull/9796
[2]: https://github.com/systemd/systemd/blob/v240/NEWS#L189-L191

Change-Id: I6bc2bb691eb1a6bdbcd98843891d06fb49e6b4df