Add loopback flag to frame in RX Buffer
Currently time of transmission can be added to TXT Buffer, but during high bus load this does not mean that frame will be actually transmitted at this time.
In case of loop-back mode, it would be desirable to known when the frame was actually transmitted.
To distinguish loopback frames from regularly received frames, it would be good to add "Loopback flag" per received frame in RX Buffer.