RFC regarding rng-tools startup.

[jvonau]

Standalone instructions here: https://github.com/jvonau/rng-mod updated version of #448
Had the chance to capture the logs starting from a fresh install.
https://github.com/jvonau/rng-mod/tree/main/logs

ps -AH Before unit file

1 ? 00:00:04 systemd
126 ? 00:00:00 systemd-journal
149 ? 00:00:00 systemd-udevd
319 ? 00:00:00 systemd-timesyn
354 ? 00:00:00 avahi-daemon
374 ? 00:00:00 avahi-daemon
355 ? 00:00:00 cron
357 ? 00:00:00 alsactl
364 ? 00:00:00 rsyslogd
368 ? 00:00:00 dbus-daemon
373 ? 00:00:00 systemd-logind

377 ? 00:00:00 rngd

375 ? 00:00:00 thd
382 ? 00:00:00 wpa_supplicant
484 ? 00:00:00 wpa_supplicant
500 ? 00:00:00 hciattach
505 ? 00:00:00 bluetoothd
570 ? 00:00:00 dhcpcd

ps -AH After installing unit file

1 ? 00:00:04 systemd

124 ? 00:00:00 rngd

140 ? 00:00:00 systemd-journal
160 ? 00:00:00 systemd-udevd
334 ? 00:00:00 systemd-timesyn
371 ? 00:00:00 systemd-logind
373 ? 00:00:00 cron
382 ? 00:00:00 dbus-daemon
396 ? 00:00:00 wpa_supplicant
397 ? 00:00:00 avahi-daemon
425 ? 00:00:00 avahi-daemon
408 ? 00:00:00 rsyslogd
413 ? 00:00:00 alsactl
417 ? 00:00:00 thd
490 ? 00:00:00 wpa_supplicant
509 ? 00:00:00 hciattach
513 ? 00:00:00 bluetoothd
578 ? 00:00:00 dhcpcd

That is about the quickest startup point without embedding rng-tools into the initrd.img. I'm not really sure where upstream this should be filed with rnd-tools or systemd but it works for me. Might help with low entropy situations with no harm otherwise.
Have the before and after dmesg, syslog, and journalctl if you want look but in think the ps output illustrates the change with less info to look at.