Minor fixes for RTC and some additional functions

This series of commits is to the rtc library  (both common and f4-specific)

1. The first commit fixes a few minor typos in the file.

2. The second commit looks crazy, but it's all just formatting.  Lots of additional comments and a conversion of tabs to spaces (courtesy of my editor).

3. The third commit has additional functions specifically for the f4.

4. The fourth commit has functions applicable to all devices in the series.