Merge pull request #3402 from kaspar030/simplify_shell

boards/chronos/stdio.c

examples/default/Makefile

examples/default/main.c

examples/gnrc_networking/Makefile

examples/gnrc_networking/main.c

sys/include/shell.h

sys/shell/shell.c

tests/driver_at86rf2xx/Makefile

tests/driver_at86rf2xx/main.c

tests/driver_kw2xrf/Makefile

tests/driver_kw2xrf/main.c

tests/driver_netdev_eth/main.c

tests/driver_nrf24l01p_lowlevel/main.c

tests/driver_nrfmin/Makefile

tests/driver_nrfmin/main.c

tests/driver_pcd8544/Makefile

tests/driver_pcd8544/main.c

tests/driver_xbee/main.c

tests/periph_gpio/Makefile

tests/periph_gpio/main.c