bitbake: knotty: Drop task prefix of PLAIN log messages

To quote Paul:

Not that long ago we added a prefix to logged messages to allow us to see
where the message has been generated (recipe / task). This is undoubtedly
useful for errors and warnings, however, I'm not sure it's really appropriate
for bb.plain(). As an example, see the output for -c listtasks now:

...
NOTE: Executing RunQueue Tasks
nodejs-native-4.5.0-r0 do_listtasks: do_addto_recipe_sysroot
nodejs-native-4.5.0-r0 do_listtasks: do_build                      Default task for a recipe - depends on all other normal tasks required to 'build' a recipe
nodejs-native-4.5.0-r0 do_listtasks: do_checklicense
nodejs-native-4.5.0-r0 do_listtasks: do_checklicenseall
nodejs-native-4.5.0-r0 do_listtasks: do_checkpkg
...

This patch excludes PLAIN messages from this prefixing making the log output
neater.

[YOCTO #11457]

(Bitbake rev: 4a14b44b3e4fad3a3e5e53461aa8ba9929a515b8)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/ui/knotty.py

@@ -472,7 +472,7 @@ def main(server, eventHandler, params, tf = TerminalFilter):
                         continue
 
                     # Prefix task messages with recipe/task
-                    if event.taskpid in helper.running_tasks:
+                    if event.taskpid in helper.running_tasks and event.levelno != format.PLAIN:
                         taskinfo = helper.running_tasks[event.taskpid]
                         event.msg = taskinfo['title'] + ': ' + event.msg
                 if hasattr(event, 'fn'):