Merge pull request #4862 from haukepetersen/opt_periph_gpio_mode

boards/arduino-due/board.c

boards/f4vi1/board.c

boards/fox/board.c

boards/frdm-k64f/board.c

boards/iotlab-m3/board.c

boards/iotlab-m3/include/gpio_params.h

boards/limifrog-v1/board.c

boards/limifrog-v1/include/periph_conf.h

boards/mulle/board.c

boards/nrf51dongle/board.c

boards/nrf52dk/include/gpio_params.h

boards/nucleo-f091/board.c

boards/nucleo-f103/board.c

boards/nucleo-f303/board.c

boards/nucleo-f334/board.c

boards/nucleo-f401/board.c

boards/nucleo-l1/board.c

boards/nucleo-l1/include/periph_conf.h

boards/openmote-cc2538/board.c

boards/pba-d-01-kw2x/board.c