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)

No fixed value for USB power current

This should save some cycles inside ISR Handler.

Change the way ISRs are stored. Store now in a list on a ‘first come
first served basis’, the first interrupt that gets attached will be
served first. This improves the speed of the ISR calling from about 6us
to 2us for a single interrupt on extint14

See #200

Solve #199

Fix #223

See https://github.com/arduino/Arduino/issues/4525