Merge pull request #4431 from haukepetersen/opt_periph_dac

boards/f4vi1/include/periph_conf.h

boards/msbiot/include/periph_conf.h

boards/nucleo-f401/include/periph_conf.h

boards/stm32f4discovery/include/periph_conf.h

cpu/stm32f4/include/periph_cpu.h

cpu/stm32f4/periph/dac.c

drivers/include/periph/dac.h

sys/analog_util/Makefile

sys/analog_util/adc_util.c

sys/analog_util/dac_util.c

sys/include/analog_util.h

tests/periph_dac/README.md

tests/periph_dac/main.c