I had the same issue, the “solution” was already the original code, so I had to look for a fast and simple solution by myself.
You can inject a bit of javascript in header.php to toggle the ul menu tag:
– go to the wp editor for header.php
– find
<button class="menu-toggle"><?php _e( 'Menu', 'twentytwelve' ); ?></button>
– just add a bit of javascript as follows:
<button class="menu-toggle" onclick="el = document.getElementById('menu-menu-principal'); el.style.display = (el.style.display != 'inline' ? 'inline' : 'none');"><?php _e( 'Menu', 'twentytwelve' ); ?></button>
Cheers!
vincentperrin.com