SAMD CORE 1.6.10

* The linker script must define `__text_start__` symbol at the beginning of .text section.
  This symbol is used to automatically determine the bootloader size.

SAMD CORE 1.6.6

* `digitalPinToInterrupt` #define moved to Arduino.h, variant.h must no longer define it.

SAMD CORE 1.6.3

* Timer for pin PWM selected based on value of `PIN_ATTR_TIMER_ALT` or `PIN_ATTR_TIMER`.
  Prior to this "pin type" (ulPinType) was used.
