RTT_COMMAND: Allow no down-channels

[laurensmiers]

If no down-channel can be found, the target only supports printing RTT messages.

Before this commit, this target-configuration was only possible to do when supplying a log file. Then commands were ignored and only the up-channel was written to the file. If no log-file was supplied, we detected the missing down-channels and returned an error, never starting the viewer_loop so no up-channel messages were printed on stdout.

With this commit, instead of returning an error when no down-channels are configured, we inform the user and start the viewer_loop with a down-channel (parameter down_chan) with value 'None'.

Inside the viewer_loop we detect the None-value of down_chan, drop the user supplied command and continue our viewer_loop.

[laurensmiers]

Not sure what is stopping the jobs from starting? If I need to do something, do tell.