Adc example now works. Rcc freq setting fixed.

Adc example works with adc and usart2.
Rcc: pllsrc defines added.
     clock_scale_t with pllsrc.
     44Mhz and 64Mhz freq settings added.
     New function "rcc_wait_for_osc_not_ready added.
     Frequency setting function improved.
     Other cosmetic changes.