Merge pull request #10409 from aabadie/pr/boards/nrf51dk

boards/nrf51dk/Makefile

boards/nrf51dk/Makefile.dep

boards/nrf51dk/Makefile.features

boards/nrf51dk/Makefile.include

boards/nrf51dk/board.c

boards/nrf51dk/doc.txt

boards/nrf51dk/include/board.h

boards/nrf51dk/include/gpio_params.h

boards/nrf51dk/include/periph_conf.h

cpu/nrf51/include/cpu_conf.h

examples/asymcute_mqttsn/Makefile

examples/default/Makefile

examples/dtls-echo/Makefile

examples/gnrc_border_router/Makefile

examples/gnrc_tftp/Makefile

examples/javascript/Makefile

examples/lua_REPL/Makefile

examples/lua_basic/Makefile

examples/posix_sockets/Makefile

tests/gnrc_netif/Makefile