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.