stm32f1: usart hal driver with proper BR calculation
stm32f1: usart hal driver with proper BR calculation

hackndev/zinc,richo/zinc,mcoffin/zinc,richo/zinc,ntwyman/zinc,mcoffin/zinc,ntwyman/zinc,ntwyman/zinc,hackndev/zinc,mcoffin/zinc,ntwyman/zinc,ntwyman/zinc,mcoffin/zinc,mcoffin/zinc,richo/zinc,richo/zinc,hackndev/zinc,hackndev/zinc,hackndev/zinc,richo/zinc