Damien George 46e5d6b889 stm32/rtc: Add auto-LSE-bypass detection with fallback to LSE then LSI.
If MICROPY_HW_RTC_USE_BYPASS is enabled the RTC startup goes as follows:
- RTC is started with LSE in bypass mode to begin with
- if that fails to start (after a given timeout) then LSE is reconfigured
  in non-bypass
- if that fails to start then RTC is switched to LSI
2019-04-11 12:09:21 +10:00
..