The exported variable is used by banzai() to avoid erasing any bootloader part

This commit enables PWM on pin 11 instead of pin A4 on MKR1000 improving UNO pinout compatibility

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);

Fix #126

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

The variant's file compliance is now determined using a define named

ARDUINO_SAMD_VARIANT_COMPLIANCE

that refer to the minimum core version required for this variant file
to work.

Pins A1/A2 uses TC4 to generate PWM, the same used in Servo library,
so both cannot work together.

The attribute ATTR_ANALOG is needed to select DAC, so it has been
removed from pins A1..A5 that doesn't have a DAC.

The selection of the correct timer peripheral is now
decided through the PIN_ATTR_{TIMER/TIMER_ALT} attribute.

See #30

Now PA14 is connected to Arduino Zero digital pin 2(NO-PWM) and PA08 is connected to digial pin 4(PWM)
Also connected PA13 to Arduino Zero ATN pin