boards: fix doxygen grouping

boards/chronos/board_init.c

boards/chronos/drivers/battery.c

boards/chronos/drivers/buzzer.c

boards/chronos/drivers/display.c

boards/chronos/drivers/display_putchar.c

boards/chronos/stdio.c

boards/common/arduino-mkr/board.c

boards/common/atmega/board.c

boards/common/msba2/drivers/include/sht11-board.h

boards/common/nrf52xxxdk/board.c

boards/common/stm32f103c8/include/periph_conf.h

boards/frdm-k22f/include/gpio_params.h

boards/frdm-kw41z/include/adc_params.h

boards/frdm-kw41z/include/gpio_params.h

boards/msba2/board_init.c

boards/native/board_init.c

boards/native/drivers/native-led.c

boards/pba-d-01-kw2x/board.c

boards/remote-pa/board.c