boards/arduino-zero: add initial support

boards/arduino-zero/Makefile

boards/arduino-zero/Makefile.dep

boards/arduino-zero/Makefile.features

boards/arduino-zero/Makefile.include

boards/arduino-zero/board.c

boards/arduino-zero/dist/openocd.cfg

boards/arduino-zero/include/arduino_board.h

boards/arduino-zero/include/arduino_pinmap.h

boards/arduino-zero/include/board.h

boards/arduino-zero/include/gpio_params.h

boards/arduino-zero/include/periph_conf.h

cpu/sam21_common/include/sam0.h

cpu/samd21/ldscripts/samd21g18a.ld

tests/unittests/Makefile