It can be that LSEON and LSERDY are set yet the RTC is not enabled (this
can happen for example when coming out of the ST DFU mode on an F405 with
the RTC not previously initialised). In such a case the RTC is never
started because the code thinks it's already running. This patch fixes
this case by always checking if RTCEN is set when booting up (and also
testing for a valid RTCSEL value in the case of using an LSE).