handleIRQ() hardcoded USART1 when clearing the RXNE pending bit, so on
boards where the repeater/display UART is not USART1 (Nucleo, Discovery,
F767, UART5-based boards) the flag was cleared on the wrong peripheral.
Use the m_usart member, matching the TXE handling below.
Also fix flush(): it passed a flag constant to USART_GetITStatus and
span while TXE was set, which returns immediately since TXE indicates
the data register is already empty. Wait for the software TX FIFO to
drain and then for the TC flag, which indicates the shift register has
actually emptied.
Clint Chance
Geoffrey Merck