Uart RX Threading Issue

I had the same bug in my UartReceiveByte function as I did in the
UartTransmitByte function. I don't know what I was thinking when I
declared that isEmpty variable. It should be good now.
Assuming Manual Control of the Cursor

I have implemented cursor movement. When the cursor leaves the bounds
of the screen it is indicated in the border of the game.

I also fixed another similar bug in the UART driver relating to
threading.