NSH term: Only time out if no arming information is available, if arming...

... information is available abort if unconnected on arming.

Do not merge yet, untested. We might also want to consider to disconnect USB, need to validate this.

This addresses this usability issue: https://github.com/mavlink/qgroundcontrol/issues/940

It will however still mean that if you arm first and then try to connect USB it won't work - but that seems far more intuitive.