drivers: adapt ng_at86rf2xx to new auto_init_ng_netif scheme

boards/iot-lab_M3/auto_init_ng_netif/Makefile

boards/iot-lab_M3/auto_init_ng_netif/netif_board.c

boards/iot-lab_M3/include/ng_at86rf2xx_params.h

boards/samr21-xpro/include/ng_at86rf2xx_params.h

drivers/include/ng_at86rf2xx.h

sys/auto_init/auto_init.c

sys/auto_init/netif/auto_init_ng_at86rf2xx.c

tests/driver_at86rf2xx/Makefile

tests/driver_at86rf2xx/auto_init_ng_netif/Makefile

tests/driver_at86rf2xx/auto_init_ng_netif/netif_app.c

tests/driver_at86rf2xx/main.c

tests/driver_at86rf2xx/ng_at86rf2xx_params.h