boot - Multiple changes to try to make cdboot work with more bioses

boot - Multiple changes to try to make cdboot work with more bioses

* Go back to single sector read.  This seems to help gigabyte bioses
  (phenom x 6).  Multi-sector reads tend to cause reads to fail with
  a timeout for reasons unknown.

* Enable interrupts on start.

* (non operator) replace jcxz.  It's a bit unclear whether this is an
  issue or not but some assemblers do not generate the correct prefix
  for jcxz.

* Report read timeouts and delay 1 second before retrying.  At the
  very least this allows timeouts to be reported distinctly.