fix debug port use USART1 can not work bug.
Because RCC enable bit of USART1 is RCC_APB2RSTR_USART1RST (bit4)
not RCC_APB1RSTR_USART2RST (bit 17)
Merge pull request #85 from KunYi/master

discoveryf4: fix incorrect debug port assignment of USART1
root_thread adds mapping to hardware device memory