Merge pull request #5090 from haukepetersen/opt_stm_clken

boards/f4vi1/include/periph_conf.h

boards/fox/include/periph_conf.h

boards/iotlab-m3/include/periph_conf.h

boards/msbiot/include/periph_conf.h

boards/nucleo-f103/include/periph_conf.h

boards/nucleo-f401/include/periph_conf.h

boards/spark-core/include/periph_conf.h

boards/stm32f4discovery/include/periph_conf.h

cpu/stm32_common/Makefile

cpu/stm32_common/Makefile.include

cpu/stm32_common/cpu_common.c

cpu/stm32_common/include/periph_cpu_common.h

cpu/stm32f1/include/periph_cpu.h

cpu/stm32f1/periph/timer.c

cpu/stm32f4/include/periph_cpu.h

cpu/stm32f4/periph/uart.c