boards/stm32-based: adapted UART configuration

boards/f4vi1/include/periph_conf.h

boards/fox/include/periph_conf.h

boards/iotlab-common/include/periph_conf_common.h

boards/limifrog-v1/include/periph_conf.h

boards/msbiot/include/periph_conf.h

boards/nucleo-f030/include/periph_conf.h

boards/nucleo-f070/include/periph_conf.h

boards/nucleo-f072/include/periph_conf.h

boards/nucleo-f091/include/periph_conf.h

boards/nucleo-f103/include/periph_conf.h

boards/nucleo-f207/include/periph_conf.h

boards/nucleo-f303/include/periph_conf.h

boards/nucleo-f334/include/periph_conf.h

boards/nucleo-f401/include/periph_conf.h

boards/nucleo-f446/include/periph_conf.h

boards/nucleo-l1/include/periph_conf.h

boards/spark-core/include/periph_conf.h

boards/stm32f0discovery/include/periph_conf.h

boards/stm32f3discovery/include/periph_conf.h

boards/stm32f4discovery/include/periph_conf.h