boards: fix doxygen @brief in periph.conf

boards/airfy-beacon/include/periph_conf.h

boards/arduino-common/include/periph_conf.h

boards/arduino-mega2560/include/periph_conf.h

boards/avsextrem/include/periph_conf.h

boards/calliope-mini/include/periph_conf.h

boards/cc2650stk/include/periph_conf.h

boards/chronos/include/periph_conf.h

boards/ek-lm4f120xl/include/periph_conf.h

boards/f4vi1/include/periph_conf.h

boards/fox/include/periph_conf.h

boards/iotlab-a8-m3/include/periph_conf.h

boards/iotlab-m3/include/periph_conf.h

boards/limifrog-v1/include/periph_conf.h

boards/maple-mini/include/periph_conf.h

boards/mbed_lpc1768/include/periph_conf.h

boards/microbit/include/periph_conf.h

boards/msb-430/include/periph_conf.h

boards/msb-430h/include/periph_conf.h

boards/msba2/include/periph_conf.h

boards/msbiot/include/periph_conf.h