Also add Stream::find(char) API.

Port of: https://github.com/arduino/Arduino/pull/3337

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.

Conflicts:
	bootloaders/zero/drivers/cdc_enumerate.c

Tone fix for #59 and optimizations

See https://github.com/arduino/Arduino/issues/3614

This fixes issue #59 and optimize the way the prescaler is found.
Furthermore non needed instructions are not repeated.