VARIANT_COMPLIANCE_CHANGELOG

1.6.18-beta-b1 (June 26, 2018):
* MATTAIRTECH_ARDUINO_SAMD_VARIANT_COMPLIANCE is now 10618
* Added an additional PinDescription table format, which can be used to reduce code size (D11 chips only for now)
  The PinDescription table can now be 4 bytes per pin, rather than 12 bytes per pin, with little change in functionality for most.
  See the README.md in any board variant directory for more information.

1.6.8-beta-b0:
* Introduced MATTAIRTECH_ARDUINO_SAMD_VARIANT_COMPLIANCE define and VARIANT_COMPLIANCE_CHANGELOG file.
  The definitions in the variants files need the MattairTech SAMD core >= the listed version.
  The format is different than the stock Arduino SAMD core, which uses ARDUINO_SAMD_VARIANT_COMPLIANCE instead.
  If using board variant files with the old format, the new core will still read the table the old way, losing
  any new features introduced by the new column.
* The GCLKCCL column was added. It is not yet used. It will eventually support the Analog Comparators (AC),
  the Configurable Custom Logic (CCL) units of the L21 and C21, and the GCLK outputs (inputs).
* New definitions have been added for L21 and C21 support.
* The C21 has two ADC instances, which is selected in the PeripheralAttribute column.
* the timer type is now encoded in this column to support the L21 and C21, which use TC numbers starting at 0
  (rather than 3 as on the D21).
* From SAMD CORE 1.6.10 merge: The linker script must define `__text_start__` symbol at the beginning of .text section.
  This symbol is used to automatically determine the bootloader size.