wic: Integrated plugin docstrings into 'wic help plugins' output

Added mechanism to show docstrings of plugin classes as a part of
plugins help page.
For missing plugins the following warning message is shown:
<class '<plugin class spec>'> is missing docstring.

[YOCTO #7118]

(From OE-Core rev: 0997208266686473d23aed0fab58a1fd7c5d8cae)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/image/help.py

@@ -28,6 +28,7 @@
 import subprocess
 import logging
 
+from wic.plugin import pluginmgr, PLUGIN_TYPES
 
 def subcommand_error(args):
     logging.info("invalid subcommand %s" % args[0])
@@ -55,6 +56,23 @@ def wic_help(args, usage_str, subcommands):
         print(usage_str)
 
 
+def get_wic_plugins_help():
+    """
+    Combine wic_plugins_help with the help for every known
+    source plugin.
+    """
+    result = wic_plugins_help
+    for plugin_type in PLUGIN_TYPES:
+        result += '\n\n%s PLUGINS\n\n' % plugin_type.upper()
+        for name, plugin in pluginmgr.get_plugins(plugin_type).iteritems():
+            result += "\n %s plugin:\n" % name
+            if plugin.__doc__:
+                result += plugin.__doc__
+            else:
+                result += "\n    %s is missing docstring\n" % plugin
+    return result
+
+
 def invoke_subcommand(args, parser, main_command_usage, subcommands):
     """
     Dispatch to subcommand handler borrowed from combo-layer.

scripts/wic

@@ -277,7 +277,7 @@ subcommands = {
                   hlp.wic_list_help],
     "plugins":   [wic_help_topic_subcommand,
                   wic_help_topic_usage,
-                  hlp.wic_plugins_help],
+                  hlp.get_wic_plugins_help()],
     "overview":  [wic_help_topic_subcommand,
                   wic_help_topic_usage,
                   hlp.wic_overview_help],