boards/arduino-mkrzero: add initial support

boards/arduino-mkrzero/Makefile

boards/arduino-mkrzero/Makefile.dep

boards/arduino-mkrzero/Makefile.features

boards/arduino-mkrzero/Makefile.include

boards/arduino-mkrzero/board.c

boards/arduino-mkrzero/dist/bossac

boards/arduino-mkrzero/dist/bossac_osx

boards/arduino-mkrzero/dist/flash.sh

boards/arduino-mkrzero/include/arduino_board.h

boards/arduino-mkrzero/include/arduino_pinmap.h

boards/arduino-mkrzero/include/board.h

boards/arduino-mkrzero/include/gpio_params.h

boards/arduino-mkrzero/include/periph_conf.h