bitbake: implement command to find configuration files for a config variable

Some configuration variables (MACHINE, MACHINE-SDK and DISTRO) set which
confguration files bitbake should use.
The added command , findConfigFiles, enables a UI to query which files are
suitable values for a specified parameter.

Signed-off-by: Joshua Lock <josh@linux.intel.com>

bitbake/lib/bb/event.py

@@ -359,6 +359,16 @@ class TargetsTreeGenerated(Event):
         Event.__init__(self)
         self._model = model
 
+class ConfigFilesFound(Event):
+    """
+    Event when a list of appropriate config files has been generated
+    """
+
+    def __init__(self, variable, values):
+        Event.__init__(self)
+        self._variable =  variable
+        self._values = values
+
 class MsgBase(Event):
     """Base class for messages"""