Add minimum battery voltage to consider a battery charged.

If you switch off the system while the battery is stressed,
the voltage can easily be two volts lower than without any load.
-> We detected the battery as charged on the next startup.

Prevent this by defining a minimum "charged" voltage for the battery.

New config.h entry:
   const byte battery_charged_min_voltage = 20;

Adjust the value to roughly the voltage value of your
fully charged battery minus a safety margin.