Disallow USB connections with t2 root command (#673)

[wyze]

Fixes #673

This will hide the flags if someone runs t2 root --help but doesn't disable them completely. I saw at least 2 options here, either error out and don't attempt a connection, or force the use of LAN connection and give the user a warning. I chose the 2nd route as I think it is a better user experience. But I am up for discussions on it.

Current output:

❯ t2 root --usb
WARN You are trying to connect to Tessel via USB, but this command only works with Wifi.
WARN I will use Wifi to try and look for your Tessel.
INFO Looking for your Tessel...
INFO Connected to t2-testing.
INFO Starting SSH Session on Tessel. Type 'exit' at the prompt to end.

[rwaldron]

LGTM

@HipsterBrown @tikurahul R?

[tikurahul]

LGTM