Add chrony support to 430_prepare_timesync.sh

[N3WWN]

• rear version (/usr/sbin/rear -V): 2.3 and higher
• OS version (cat /etc/rear/os.conf or lsb_release -a): n/a
• Brief description of the issue:
  • script rescue/default/430_prepare_timesync.sh needs an update for chronyd

This is a carry-over of additional work from #1598

[gdha]

@N3WWN Are you still working on this?

[N3WWN]

Yes, I'm finishing it up and am hoping to get a PR submitted for this early next week.

I just got time to revisit it about a week ago and, as of right now, a colleague is testing it for me.

[jsmeix]

With #1739 merged,
this issue should be fixed.