shell: added RPL route shell command
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
added NORETURN to `sched_task_exit()`
added NORETURN to `cpu_switch_context_exit()`
added NORETURN to `sched_task_exit()`