Sync RTC with PiJuice on boot

[nicolas-f]

https://github.com/PiSupply/PiJuice/tree/master/Software#sync-rtc-time-at-boot

Sync RTC time at boot

When the Raspberry Pi shutsdown and then reboots you must copy the RTC time back to the system clock at boot and you can do this in /etc/rc.local with sudo hwclock -s.

Note: This assumes that your PiJuice has sufficient power from the battery to keep the simulated RTC running in the PiJuice microcontroller while the Pi is shut down.