wic: Add command to list available source plugins

Add a 'wic list source-plugins' command enabling users to get a list
of valid partition --sources.  This is useful not only for determining
sources to use in .wks partition statements, but also for making sense
of errors in .wks partition processing.

(From OE-Core rev: 37db9423e7872012fabde67e8858db5b512832f6)

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/mic/plugin.py

@@ -117,6 +117,22 @@ class PluginMgr(object):
 
         return pluginbase.get_plugins(ptype)
 
+    def get_source_plugins(self):
+        """
+        Return list of available source plugins.
+        """
+        plugins_dir = self._build_plugin_dir_list(self.plugin_dir, 'source')
+
+        self.append_dirs(plugins_dir)
+
+        plugins = []
+
+        for _source_name, klass in self.get_plugins('source').iteritems():
+            plugins.append(_source_name)
+
+        return plugins
+
+
     def get_source_plugin_methods(self, source_name, methods):
         """
         The methods param is a dict with the method names to find.  On