My configuration

gistfile1.aw

<?php
class Configuration implements ConfigurationInterface
{
    /**
     * {@inheritDoc}
     */
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('kp_media');
        $rootNode->children()
            ->arrayNode('linkmap')
                ->prototype('array')


                ->useAttributeAsKey('linkmap')
                    ->prototype('variable')
                ->end()
            ->end()

            ->end()
            ->arrayNode('uploader')->children()
                    ->scalarNode('media_path')->defaultValue('media')->end()
                ->end()->end()
            ->arrayNode('imager')->isRequired()
                ->children()->arrayNode('options')->children()
                    ->scalarNode('lib')->defaultValue('gd')->end()
                    ->arrayNode('thumbnails')
                                ->prototype('array')
                                    ->useAttributeAsKey('name')
                                    ->prototype('variable')->end()
                           ->end()
                        ->end()
                    ->end()
 //           ->end()
        ->end();


        // Here you should define the parameters that are allowed to
        // configure your bundle. See the documentation linked above for
        // more information on that topic.

        return $treeBuilder;
    }
}