Allow controlling USB/UART log output independently.
Log when CAN bus is quiet and we expect to go to sleep shortly.
Only clear LPC17xx USB endpoint we actually sent data.
Clear predefined OBD-II message status for each functional test.
Use larger buffers for outgoing data and more retries before dropping.
Set payload format back to JSON when done with func tests.

This clears the way for the BT tests to run after USB, since protobuf
commands aren't working over BT yet.