To be compliant with the one sent by the AVR core

Signed-off-by: Thibaut VIARD <aethaniel@sam-geek.org>

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.

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 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.

The shortcut has some issues:

- sometimes it fails when sending an odd number of bytes (may be
  due to memory alignment?)
- the data pointer should point to "stable" data (and this is not
  guaranteed by caller, it may be some sort of temporary buffer)
- the SRAM is not guaranteed to start at 0x20000000

All the above problems must be properly fixed before reenabling
this part

* fix in_endpoint buffer size (was too big)
* use the same EP for CDC_ENDPOINT_OUT and CDC_ENDPOINT_IN (different buffers)
* fix CDC_ENDPOINT_IN ep type

This way all the 64 bytes available can be filled up

Fixes #30

calling digitalWrite with any value different from 0 will end
in performing digitalWrite(HIGH)