How to show the primary navigation with dropdown elements in the header block?

I found a way with a custom module:

function bootstrap_header_block_view_system_header_alter(&$data, $block) {

$data['content'] = bootstrap_header_header_block_content();

}


function bootstrap_header_header_block_content() {


$settings = array(

'logo' => TRUE,

'site_name' => TRUE,

'site_slogan' => TRUE,

);


$settings_block = array(

'menu_name' => 'main-menu',

'style' => 'dropdown',

'level' => 1,

'depth' => 0,

);


module_load_include('inc', 'system', 'system.menu');

$block = system_menu_block_build($settings_block);


$site_config = config('system.core');

$variables['logo'] = $settings['logo'] ? backdrop_get_logo() : NULL;

$variables['logo_attributes'] = $settings['logo'] ? array_diff_key(backdrop_get_logo_info(), array('path' => '')) : array();

$variables['site_name'] = $settings['site_name'] ? check_plain($site_config->getTranslated('site_name')) : '';

$variables['site_slogan'] = $settings['site_slogan'] ? filter_xss_admin($site_config->getTranslated('site_slogan')) : '';


$variables['menu'] = backdrop_render($block['content']);


return theme('header', $variables);

}