I found the following:
<?php
if ( has_nav_menu( 'primary' ) ) {
wp_nav_menu( array(
'container' => 'div',
'container_class' => 'nav-collapse collapse',
'theme_location' => 'primary',
'menu_class' => 'nav visible-desktop',
'walker' => new Bootstrap_Walker(),
) );
}
if ( has_nav_menu( 'primary' ) ) {
wp_nav_menu( array(
'walker' => new Walker_Nav_Menu_Dropdown(),
'theme_location' => 'primary',
'items_wrap' => '<div id="mobile-menu" class="pull-right visible-tablet visible-phone mobile-menu"><form style="margin-top: 15px; margin-bottom: 5px;"><select style="max-width:150px;" onchange="if (this.value) window.location.href=this.value"><option>-- Menu --</option>%3$s</select></form></div>',
) );
}
?>