Convert existing cores

[matthijskooijman]

I'm opening up this issue to track official cores that have or still need to be converted to use the new API code (replacing https://github.com/arduino/Arduino/issues/5771#issuecomment-533204022)

Done:

• megaAVR (https://github.com/arduino/ArduinoCore-megaAVR)
• nRF52-mbedos (https://github.com/arduino/ArduinoCore-nRF528x-mbedos)

In progress:

• classic AVR (https://github.com/arduino/ArduinoCore-avr/tree/api)
• SAMD (https://github.com/arduino/ArduinoCore-samd/tree/api)

TODO:

• SAM(https://github.com/arduino/ArduinoCore-sam)
• ARC32 (https://github.com/arduino/ArduinoCore-arc32)