Improve boot sequence

[breakintoprogram]

Improve the sync between the eZ80 and the ESP32
Acceptance Criteria

• The eZ80 must poll the ESP32, and only start when it receives a general poll packet back
• If it times out, it must try again at a lower baud rate

[breakintoprogram]

The update version of the MOS 1.03 RC2 has also been tested on earlier versions of the VDP without the enhanced boot process. The only side-effect is 4 'c' characters appearing on screen before the MOS welcome message. This is due to corrupt characters being received from the eZ80's general poll caused by mismatched baud rates. MOS successfully fires up after that at the safe baud rate (384000).