rtc.c

Use RTC_GetTime() instead of RTC_GetSecond() because otherwise a read
immediately following the RTC alarm will return the previous value.
RTC_GetTime() will return E_BUSY a number of times before finally being
successful and returning a timestamp.  To keep the system load minimal
during this waiting, the task is put to sleep for 4ms everytime the read
fails.  This value might need further adjustment.

Signed-off-by: Rahix <rahix@rahix.de>