DISCO_F100RB board using HSI should use HSE

[RickKimball]

The STM32VL Discovery board has an external 8MHz xtal. In spite of that, the SystemClock_Config() configures itself for the HSI which leads to a less accurate SYSCLK. Please consider using the HSE with this board.

[fpistm]

@RickKimball could you submit a PR with a config using HSE ? As I do not have this one, I could not test. :'(