*: remove trailing underscores from header guards

boards/airfy-beacon/include/board.h

boards/airfy-beacon/include/periph_conf.h

boards/arduino-common/include/board.h

boards/arduino-common/include/periph_conf.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/arduino-zero/include/board.h

boards/arduino-zero/include/periph_conf.h

boards/avsextrem/include/board.h

boards/avsextrem/include/periph_conf.h

boards/avsextrem/include/smb380-board.h

boards/avsextrem/include/ssp0-board.h

boards/calliope-mini/include/mini.h

boards/cc2538dk/include/board.h

boards/cc2538dk/include/periph_conf.h

boards/cc2650stk/include/board.h

boards/cc2650stk/include/periph_conf.h

boards/chronos/drivers/include/battery.h