here and below: why define the default values for each board? Wouldn't it be much cleaner (and better scalable) to define the default values in cpu/efm32_common/cpu.c:

#ifndef CMU_xx
#define CMU_xx   CMU_xx_DEFAULT
#endif

This way a board would only need to define these values if the board actually does not use the default values...

I can also place them inline, for instance:

#ifdef _SILICON_LABS_32B_SERIES_1
static void dcdc_init(void)
{
#ifdef EMU_DCDCINIT
    EMU_DCDCInit_TypeDef init_dcdc = EMU_DCDCINIT;
#else
    EMU_DCDCInit_TypeDef init_dcdc = EMU_DCDCINIT_DEFAULT;
#endif

    EMU_DCDCInit(&init_dcdc);
}
#endif

Added benefit: cpu.c is more to the point. What would be your preference?

I personally prefer not to inline these things and have ifdef blocks in the beginning of the file. This makes the code much more readable.