Enabling Interrupts from UART 1

User programs can now wait on EVENT_UART1_TX_READY and
EVENT_UART1_RCV_READY to control communication to the train
set.

New debug macros and levels were added, DEBUG_1, DEBUG_2, DEBUG_3,
in increasing order of verboseness. The levels cascade so that
turning on DEBUG_x turns on all DEBUG_i, i < x. The debug level
DEBUG turns on all debug macros.