Support a predefined poti value on system startup

Works for SUPPORT_SOFT_POTI and SUPPORT_POTI_SWITCHES

New entry in config.h:
    const int poti_value_on_startup_in_watts = 0;    //poti startup value in watts
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.