core/cpu/board: moved F_CPU define to board

Building for wsn430-v1_3b .. success
Building for redbee-econotag .. success
Building for wsn430-v1_4 .. success
Building for pttu .. success
Building for telosb .. success
Building for mbed_lpc1768 .. success
Building for msb-430 .. success
Building for native .. success
Building for chronos .. success
Building for avsextrem .. success
Building for msb-430h .. success
Building for msba2 .. success