Handle broken RTC

[troglobit]

Infix needs to handle platforms with broken RTC better. "Broken" in this case means a non-battery backed RTC that at power-on start up with a completely random time in the future, which leads to skipping back in time when the NTP client synchronizes the system clock.

• On platforms where cold start can be detected, and have this problem, we should clear the RTC
• On platforms where cold start cannot be detected, we should disable the RTC
  • Disable the driver, or
  • Disable the node in the device tree
• When the system starts, detecting a missing /dev/rtc (and others) should restore from a file instead
• Periodically (every 11 mins, like the kernel) save system time to a file

The latter two can be handled by extending the RTC plugin in Finit, see finit-project/finit#418