STM32F1xx RTC rtc_awake_from_off function originally reset the backup domain.
(probably because the Ref Manual erroneously required it).
This has a naughty side-effect in that unrelated user data in the BD would be wiped.
Replaced this call by clearing the RTC registers to their default values.

Tested with ET-STAMP-STM32 to verify RTC starts from power-on and reset with expected behaviour.