UART being overloaded with too many requests fix

[JensOgorek]

We had a problem with a robot, it crashed with a lot of checksum mismatch outputs.
The traceback pointed towards the UART.

It seems the UART was overloaded with too many requests that flooded in while it was still sending.

I recreated the issue with two esps. The expander esp had 21 input modules to create a higher load on the connection.
After both were running, I could trigger the issue with spamming a command like p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.info();p0.version()

After spamming it for multiple seconds, it resulted in the same behavior the robot displayed.

The fix is to wait for the TX to be done sending within the echo method.