Communication protocol between CAN and CC2540 chip. [[BR]]

List of commands from CC2540 to CAN transiver: 
 1. Subscribe to a message
 2. Unsubscribe from messages
 3. List of all subscriptions 
 4. Passthrough message  

Command message structure
 1. Command code (1 byte)
 2. Payload size (1 byte)
 3. Payload (0 - 255 bytes)