classes/logging: allow shell message functions to work in devshell

classes/logging: allow shell message functions to work in devshell

Fix a regression caused by the shell message changes - if you run a
shell task's runfile, the task isn't actually running in BitBake and
thus the message FIFO won't exist - so we should just fall back to
printing the message with echo as we did before so that the run files
are still useful.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>