devtool: make plugin_init optional

So far all devtool and recipetool plugins were expected
to have plugin_init function. This function is empty in
most of plugins as they don't require initialisation.

Making plugin_init optional would allow not having empty
plugin_init in every plugin.

(From OE-Core rev: b99640c89f067866b264f4ee4030fae4c2f338c0)

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/scriptutils.py

@@ -57,7 +57,7 @@ def load_plugins(logger, plugins, pluginpath):
             plugin = load_plugin(name)
             if hasattr(plugin, 'plugin_init'):
                 plugin.plugin_init(plugins)
-                plugins.append(plugin)
+            plugins.append(plugin)
 
 def git_convert_standalone_clone(repodir):
     """If specified directory is a git repository, ensure it's a standalone clone"""