Merge branch 'zero' of github.com:arduino/ArduinoZero into zero

cores/arduino/Arduino.h

cores/arduino/SERCOM.cpp

cores/arduino/SERCOM.h

cores/arduino/WInterrupts.c

cores/arduino/WInterrupts.c.disabled

cores/arduino/WInterrupts.h

cores/arduino/WVariant.h

cores/arduino/syscalls.c

cores/arduino/wiring_analog.c

cores/arduino/wiring_constants.h

cores/arduino/wiring_digital.h

cores/validation/validation_core/build_as6/test.cppproj

cores/validation/validation_core/test.cpp

libraries/SPI/SPI.cpp

variants/arduino_zero/variant.cpp

variants/arduino_zero/variant.h