Enable RTC sync in Chrony

[troglobit]

Since the system clock is only saved to the RTC on controlled shutdown/reboot, it would be very useful to have the NTP client update the RTC when enabled.

Chrony has both rtcsync and rtcautotrim <threshold>, both seem like reasonable candidates.

[troglobit]

We already have rtcsync enabled by default.

infix/board/common/rootfs/etc/chrony/chrony.conf

Lines 21 to 23 in fbddedb

	# Enables kernel synchronisation (every 11 minutes) of the real-time
	# clock. Note that it can’t be used along with the 'rtcfile' directive.
	rtcsync