slip: port to be used with netdev

Makefile.dep

dist/tools/tunslip/README.md

drivers/Makefile.dep

drivers/include/net/netdev.h

drivers/include/slipdev.h

drivers/slipdev/Makefile

drivers/slipdev/slipdev.c

examples/gcoap/Makefile.slip

examples/gcoap/slipdev_params.h

examples/gnrc_border_router/Makefile

examples/gnrc_border_router/README.md

sys/auto_init/auto_init.c

sys/auto_init/netif/auto_init_slipdev.c

sys/include/net/gnrc/netdev/raw.h

sys/include/net/gnrc/slip.h

sys/net/gnrc/Makefile

sys/net/gnrc/link_layer/netdev/gnrc_netdev_raw.c

sys/net/gnrc/link_layer/slip/gnrc_slip.c

tests/slip/Makefile

tests/slip/slipdev_params.h