Tone support for Arduino Due

[jgromes]

Arduino Due currently has no internal support for tone(), which is required for AFSK. There's an open PR in the core (arduino/ArduinoCore-sam#108), once it's merged, RADIOLIB_TONE_UNSUPPORTED flag can be removed from build options.

[jgromes]

once it's merged, RADIOLIB_TONE_UNSUPPORTED flag can be removed from build options.

No longer true since RadioLib now supports CubeCell too (which also cannot do tone).