Merge pull request #3101 from gebart/pr/warning-fixes

cpu/cortexm_common/include/core_cmFunc.h

cpu/k60/devio/devio-null.c

cpu/k60/devio/devio-uart.c

cpu/k60/syscalls.c

cpu/kinetis_common/hwtimer_arch.c

cpu/kinetis_common/i2c.c

cpu/kinetis_common/random_rnga.c

cpu/kinetis_common/random_rngb.c

cpu/kinetis_common/spi.c

cpu/kinetis_common/timer.c

cpu/stm32f4/periph/i2c.c

cpu/stm32f4/periph/random.c

cpu/stm32f4/periph/spi.c

drivers/at86rf231/at86rf231.c

sys/newlib/syscalls.c