The STM32F4Discovery board doesn't come with a Low speed external oscillator...

The STM32F4Discovery board doesn't come with a Low speed external oscillator so the default LSE source for the RTC doesn't work.

In stm32_rtcc.c the up_rtcinitialize() logic doesn't work with the LSI. The check on RTC_MAGIC on the BK0R register lead to rtc_setup() call that rightfully enables the lsi clock; but the next times, when the rtc is already setup, the rtc_resume() call does NOT start the lsi clock!

The right place to put LSE/LSI initialisation is inside stm32_stdclockconfig() in stm32fxxxxx_rcc.c.  Doing this I checked the possible uses of the LSI and the LSE sources: the LSI can be used for RTC and/or the IWDG, while the LSE only for the RTC (and to output the MCO1 pin)..

This change is not verifed for any other platforms.

From Leo Aloe3132