Get wrt160nl platform building

- Specify UART_FIFO_LEN and UART_CSR_SPACED in xinu.conf instead of
  now-nonexistent uart-ns16550a driver
- Define NPOOL in xinu.conf
- In include/bufpool.h, set dummy POOL_{MIN,MAX}_BUFSIZE, POOL_MAX_NBUFS if not
  defined in xinu.conf and NPOOL == 0.
- Fix wrt160nl/interrupt.h (it needed to include the default MIPS header)