boards/arduino-mkr: move shared code to common/arduino-mkr

boards/arduino-mkr1000/Makefile

boards/arduino-mkr1000/Makefile.dep

boards/arduino-mkr1000/Makefile.features

boards/arduino-mkr1000/Makefile.include

boards/arduino-mkrzero/Makefile

boards/arduino-mkrzero/Makefile.dep

boards/arduino-mkrzero/Makefile.features

boards/arduino-mkrzero/Makefile.include

boards/common/arduino-mkr/Makefile

boards/common/arduino-mkr/Makefile.dep

boards/common/arduino-mkr/Makefile.features

boards/common/arduino-mkr/Makefile.include

boards/common/arduino-mkr/board.c

boards/common/arduino-mkr/include/arduino_board.h

boards/common/arduino-mkr/include/arduino_pinmap.h

boards/common/arduino-mkr/include/board_common.h

boards/common/arduino-mkr/include/gpio_params.h

boards/common/arduino-mkr/include/periph_conf.h