The UART has a receive buffer which will fill up when it receives bytes.
Once the buffer is full, it raises the RX flag until the value is read
by the cpu.
Currently an interrupt is triggered any time there is any activity on
the UART_RXD line, but later it will only trigger once there is data
ready to be read.
Added new signal tx_flag, which indicates whether the transmitter is
ready for new data.
Added status register, which when read will return the tx_flag bit, as
well as others that can be implemented.
Added new state IDLE, which resets the TX flag and allows new data to be
written.
Added code to allow for different baud rates, though it is still fixed
currently.
