Merge branch 'archive_file_path-backwards-compatibility' of https://github.com/sandeepmistry/ArduinoCore-samd

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);

Port of https://github.com/arduino/Arduino/pull/4737

Fix #124

Fix #126

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.

The pin mapping is still subject to change, better to not distribute
files that are not final.

USBHost used to have an init() method that now is incorporated in
Init(). The leftover init() calls in the library were calling
the init() function in the variant files instead (that has nothing
to do with the USBHost library).

Removing the calls to init() fix it.

Fix #115