no setting – you would need to start with creating a child theme of Twenty Twelve; http://codex.wordpress.org/Child_Themes
then edit a copy of header.php within the child theme, and move this section:
<nav id="site-navigation" class="main-navigation" role="navigation">
<h3 class="menu-toggle"><?php _e( 'Menu', 'twentytwelve' ); ?></h3>
<div class="skip-link assistive-text"><a href="#content" title="<?php esc_attr_e( 'Skip to content', 'twentytwelve' ); ?>"><?php _e( 'Skip to content', 'twentytwelve' ); ?></a></div>
<?php wp_nav_menu( array( 'theme_location' => 'primary', 'menu_class' => 'nav-menu' ) ); ?>
</nav><!-- #site-navigation -->
to just above this line:
</header><!-- #masthead -->
Thanks a bunch, I popped this change right into my child theme and it worked without (it appears) breaking the mobile-ready look. Thanks!