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
make: move exported make vars to one place
make: add a short overview how the exported vars are used
make: put the exports into a file on its own
make: don't use export on example makefiles