SAMD CORE 1.6.10

* the linker script is use should define __text_start__ symbol at the beginning of .text section

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