boards/cpu/drivers/sys: use generic stdio_ if

boards/bluepill/include/board.h

boards/common/arduino-atmega/include/board_common.h

boards/common/iotlab/include/board_common.h

boards/common/msb-430/board_init.c

boards/common/silabs/drivers/bc/bc.c

boards/common/wsn430/board_init.c

boards/hifive1/Makefile.include

boards/jiminy-mega256rfr2/include/board.h

boards/maple-mini/include/board.h

boards/mega-xplained/include/board.h

boards/mulle/include/board.h

boards/opencm904/include/board.h

boards/ruuvitag/Makefile.include

boards/stm32mindev/include/board.h

boards/telosb/board.c

boards/telosb/include/board.h

boards/thingy52/Makefile.include

boards/waspmote-pro/include/board.h

boards/z1/board.c

cpu/atmega_common/Makefile.include