op-mode: T6715: manually changing time/date is not synced into hardware clock

When not using NTP and adjusting the current system time/date using set date
the time is not saved across a reboot into the hardware RTC. This commit
explicitly syncs the current time after a change into the systems RTC.

Most routers do not run without NTP (which is even a VyOS default) so the
priority is pretty low.

(cherry picked from commit 835126e249c1a8b7ae87ac169a8eb9d2df979249)