Communication between EPSs via CAN? (Core ↔ Plexus)

[JensOgorek]

This could be the next step after #158 ("Allow Plexus to work as a standalone node").
The idea is to replace UART with RS485 by using CAN for communication between the Plexus and the core, sharing the same bus as the motors.
This would also free up one of the three module slots on a Plexus Mini, since the RS485 module would not be required anymore.

Open questions / challenges:

• Startscript transfer: How to send the Lizard start script over CAN?
• Debugging: Without UART, how do we handle debugging? (USB access as fallback?)
• Message format: CAN does not support full string messages. Both communication partners must agree on the CAN message structure and its meaning beforehand.
• OTA: OTA (over the wire) updates must also work over can.
• Spam: Will this result in too much spam to handle for the CAN Bus? Will the motors be negatively effected by this?