gnrc: make all gnrc modules sub-modules of gnrc

Makefile.dep

Makefile.include

Makefile.pseudomodules

boards/iot-lab_M3/Makefile.dep

boards/mulle/Makefile.dep

boards/native/Makefile.dep

boards/pba-d-01-kw2x/Makefile.dep

boards/samr21-xpro/Makefile.dep

cpu/msp430-common/include/cpu_conf.h

cpu/native/dev_eth_tap/dev_eth_tap.c

cpu/nrf51/include/nrfmin.h

cpu/nrf51/radio/nrfmin/nrfmin.c

dist/tools/sniffer/README.md

dist/tools/tunslip/README.md

drivers/at86rf2xx/at86rf2xx.c

drivers/at86rf2xx/at86rf2xx_netdev.c

drivers/at86rf2xx/include/at86rf2xx_netdev.h

drivers/include/at86rf2xx.h

drivers/include/kw2xrf.h

drivers/include/xbee.h