[NUCLEO_L152RE] Add LSE configuration for RTC and...

... remove trailing spaces, typo corrections
[NUCLEO_F103RB] Add LSE configuration for RTC
[NUCLEO_F103RB] Remove trailing spaces, typo corrections
Merge pull request #281 from bcostm/master

[NUCLEO_L152RE/F103RB] Add LSE configuration for RTC