Merge pull request #8549 from haukepetersen/opt_boards_stm32-clk-f4-180

boards/common/nucleo/Makefile.include

boards/common/stm32/doc.txt

boards/common/stm32/include/cfg_spi_divtable.h

boards/common/stm32/include/f4/cfg_clock_168_16_0.h

boards/common/stm32/include/f4/cfg_clock_168_8_1.h

boards/common/stm32/include/f4/cfg_clock_180_8_1.h

boards/f4vi1/Makefile.include

boards/f4vi1/include/periph_conf.h

boards/msbiot/Makefile.include

boards/msbiot/include/periph_conf.h

boards/nucleo-f429zi/include/periph_conf.h

boards/nucleo-f446re/include/periph_conf.h

boards/nucleo-f446ze/include/periph_conf.h

boards/stm32f4discovery/Makefile.include

boards/stm32f4discovery/include/periph_conf.h