[Paciente8159/uCNC PR#72] v1.3 – Added support for MCU SAMD21 (Arduino Zero/M0 boards)

– added support for MCU SAMD21 (Arduino Zero/M0 boards) with minor limitations. These are:
– Interrupt driven inputs not implemented (only working via soft pooling)
– Serial port COM not implemented. (limited to Virtual COM port via USB)
– Analog inputs not implemented. (this feature is outside of the Grbl scope)