boards: fix Doxygen group directives

boards/acd52832/include/board.h

boards/airfy-beacon/include/board.h

boards/arduino-duemilanove/include/board.h

boards/arduino-mega2560/include/board.h

boards/arduino-mkr1000/include/board.h

boards/arduino-mkrzero/include/board.h

boards/arduino-uno/include/board.h

boards/arduino-zero/include/board.h

boards/avsextrem/include/board.h

boards/b-l072z-lrwan1/include/board.h

boards/b-l475e-iot01a/include/board.h

boards/bluepill/include/board.h

boards/calliope-mini/include/board.h

boards/cc2538dk/include/board.h

boards/cc2650stk/include/board.h

boards/chronos/include/board.h

boards/ek-lm4f120xl/include/board.h

boards/f4vi1/include/board.h

boards/feather-m0/include/board.h

boards/fox/include/board.h