The EFM32 uses the provided _SILICON_LABS_32B_SERIES_0 and
_SILICON_LABS_32B_SERIES_1 definitions to enable or disable certain
code. With the introduction of new MCUs, there is also the
_SILICON_LABS_32B_SERIES_2 definition.
This PR ensures that the defines are explicit, and that #else
statements don't target the wrong series.
- Removed stdio_init() from newlib's _init(), as this is too late in the boot
process to allow DEBUG()ing during periph_init()
- Added stdio_init() to the various cpu_init() routines of the ARM CPUs just
before periph_init()
