Merge pull request #4577 from haukepetersen/opt_fcpu

boards/airfy-beacon/include/board.h

boards/arduino-due/include/board.h

boards/arduino-due/include/periph_conf.h

boards/arduino-mega2560/include/board.h

boards/arduino-mega2560/include/periph_conf.h

boards/avsextrem/drivers/avsextrem-ssp0.c

boards/cc2538dk/include/board.h

boards/ek-lm4f120xl/include/periph_conf.h

boards/f4vi1/include/board.h

boards/fox/include/board.h

boards/frdm-k64f/include/board.h

boards/iotlab-m3/include/board.h

boards/limifrog-v1/include/board.h

boards/mbed_lpc1768/include/board.h

boards/msbiot/include/board.h

boards/mulle/include/periph_conf.h

boards/native/include/board.h

boards/nrf51dongle/include/board.h

boards/nrf6310/include/board.h

boards/nucleo-f091/include/board.h