Battery state of charge indications are not precise

Reported by Nikolaus. Inform him when fixed.

The % display is almost correct, but calculations of remaining time are wrong, because the bq27xxx does not know exact battery capacity. It should be provided by DT through the driver to Flash inside the bq27xxx but isn't.

Another effect is that we have reduced End Of Charging voltage of the bq24297 by 50mV to 4.15V (because it could make the modem turn off due to overvoltage in some situations). This means that the battery is never charged to 100%.

This effect is not properly handled by the bq27xxx setup.