INPUT_PULDOWN was not suitable to completely discharge the input capacitor on ADC_BATTERY.
Switching to OUTPUT, LOW discharge it faster then try to read

The bootloader side will disable charging on boot by default.

it doesn't fit 8KB, use 16KB bootloader -> need to change openocd and sketch linker script

This helps removing the flicker, but if the battery is connected AFTER connecting the USB it will be unavailable.
A complete power cycle needs to be performed to recover charging/battery functionality.

HW flow control is still not working, probably some low lovel work is needed.

PMIC initialization is now handled by the GSM init() if the selected board is suitable. The only hadcoded PMIC initialization is 500mA maximum charge current.

PMIC handling needs to be decided; if we use the SDU method we could have troubles.
Probably the best way is using sercoms directly.
Anyway, the defaults SHOULD be almost ok (except the maximun charging current)