shell: added RPL route shell command
drivers: fixes to the low-level GPIO driver

- fixed doxygen comments
- added GPIO_UNDEFINED value
- added return values to all functions
Merge pull request #971 from haukepetersen/periph_gpio_fix

drivers: fixes to the low-level GPIO driver
core/cpu/board: moved F_CPU define to board
Merge pull request #872 from OlegHahm/rpl_route_shell_command

sys: net: rpl: added RPL route shell command
Merge pull request #920 from haukepetersen/fcpu_to_board

core/cpu/board: moved F_CPU define to board