PL011 TX FIFO IRQ tested successfully
Moved PL011 TX FIFO IRQ ACK into IRQ handler
Fixed bug where first character in string would be duplicated
Handled potential bug where zero-length strings would cause the ISR to read bad data
Added disable_irq() function
Disable IRQs upon entering IRQ service routine, enable before exiting
Added test to test both the UART and SYSTIMER