Port of https://github.com/arduino/Arduino/pull/4718 to SAMD core.

Fix #149

Port of https://github.com/arduino/Arduino/pull/4996

Previous 0 was always returned.

Change digitalPinToInterrupt to return pin, and use g_APinDescription[pin].ulExtInt to map pin inside attachInterrupt/detachInterrupt

This brings things inline with the atachInterrupt documentation for
usage:
attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);

Only enabled if PIN_LED_TXL or PIN_LED_RXL is defined

Port of @tico-tico’s change in
https://github.com/tico-tico/Arduino/commit/a7454b6b5c59187b95c4224aad87
bb01faa06e85 to SAMD core.

This prevents accidental overwrites of the bootloader if a NULL
pointer is used for writing by mistake.