[NES II] Add disk read timeout to bios

[JMarlin]

Description

As a user of the NES ][ it would be nice if the system were able to time out and drop to the monitor if there's no disk inserted

Acceptance criteria

• [x] Sector read code sets a failure flag and exits in all the following cases rather than retrying forever
  • [x] Checksum mismatch
  • [x] Track mismatch
• [x] ~Sector read code includes a motor on and call to motor wait at entry which checks the return flag and uses it to fail exit early~
• [x] Motor wait call returns a failure flag and exits early in the following cases:
  • [x] Reading data latch never gets a high MSB after several reads
  • [x] Reading subsequent byte never changes after several reads
• [x] BIOS boot code retries boot sector load a couple of times and finally displays a message and drops into monitor
• [x] NOS boot is not broken
• [x] NOS DIR command is not broken
• [x] NOS RUN command is not broken
• [x] DONKEY is not broken

[JMarlin]

Getting kind of frustrated with this, so here's the idea du jour: Forget the motor-on wait. Everywhere. Just base everything on retrying sector read

[JMarlin]

Wrestling with flaky-ish hardware, but this looks like it's working properly now. Got Mario to run with the current code which also handles the no-disk-at-boot condition properly, so it's functional.

I may want to add more retries to the binload routine in the future, though. And definitely will want to add proper failure handling to all of the fs and shell routines in the future.