WP - bootstrap menu generated from pages, excluding custim_meta defined

custom_menu.php

function custom_menu(){
	$o = '';
	$pid = get_queried_object_id();
	$page = get_query_var( 'paged' ) ? get_query_var( 'paged' ) : 1;

	$args = array(
		'post_type' => 'page',
		'posts_per_page' => -1,
		'post_parent' => 0,
		'order' => 'ASC',
		'orderby' => 'menu_order',
		'meta_query' => array(
			'relation' => 'OR',
			array(
				'key'     => 'not_in_menu',
				'value'   => 1,
				'compare' => 'NOT LIKE',
			),
			array(
				'key'     => 'not_in_menu',
				// 'value'   => 1,
				'compare' => 'NOT EXISTS',
			),
		),
	);
	// The Query
	$the_query = new WP_Query( $args );

	if ($the_query->have_posts()) :
		$o .= '<div id="base_menu" class="collapse navbar-toggleable-xs menu">
		<ul id="menu-top_menu" class="">';
		// The Loop
		while ( $the_query->have_posts() ) : 
			$the_query->the_post();
				$ch_args = array(
					'post_type' => 'page',
					'post_parent'=> $the_query->post->ID, 
				);
				$children = get_children($ch_args);
				$dropdown = $a_dropdown = '';
				$toggle = '';
				if(!empty($children)){
					$dropdown = 'dropdown';
					$a_dropdown = 'dropdown-toggle';
					$toggle = 'data-hover="dropdown" data-delay="0" data-close-others="true"';
				}

				$m = get_post_meta($the_query->post->ID, 'not_in_menu', true);
				$o .= '<li class="menu-item menu-item-'.$the_query->post->ID.' '.$dropdown.'" id="menu-item-'.$the_query->post->ID.'">';
					$o .= '<a '.$toggle.' class="nav-link '.$a_dropdown.'" href="'.get_permalink($the_query->post->ID).'">'.$m.get_the_title($the_query->post->ID).'</a>';

					if(!empty($children)){
						$o .= '<ul role="menu" class="dropdown-menu">';
						foreach ($children as $child) {
							$o .= '<li id="menu-item-'.$child->ID.'">
								<a class="nav-link" href="'.get_permalink($child->ID).'">'.get_the_title($child->ID).'</a>
							</li>';
						}
						$o .= '</ul>';
					}

				$o .= '</li>';
		endwhile;
		$o .= '<ul>
		</div>';
	endif;
	// Restore original Query & Post Data
	wp_reset_postdata();
	return $o;
}