Adds rtcsync directive to chrony config file

This directive enables kernel synchronisation (every 11 minutes) of the real-time clock.

It also causes chrony to update the timex sync flag, which is commonly
used to monitor clock synchronisation, e.g. via the prometheus node
exporter.

Fixes bottlerocket-os#1183

packages/chrony/chrony-conf

@@ -6,3 +6,4 @@ makestep 1.0 3
 dumponexit
 dumpdir /var/lib/chrony
 user chrony
+rtcsync