[NUCLEO_F103RB] Use HSI/LSI instead of HSE/LSE

Due to Nucleo board revision C requirements.
Merge branch 'master' of https://github.com/mbedmicro/mbed
Merge pull request #131 from bcostm/master

[NUCLEO_F103RB] Use HSI/LSI instead of HSE/LSE (board rev C constraint)