_includes/nav-menu.html

{% assign menu_pages = site.links | sort: 'menu-weight' %}
{% for menu_page in menu_pages %}
    {% assign menu_dir = menu_page.path | split: '/' | pop | join: '/' %}
    {% if menu_dir == include.path %}
        {% if menu_page.menu-title %}
            {% if menu_page.location %}
                <li>
                    <a href="{{ menu_page.location }}"
                        {% if menu_page.menu-class %}
                            class="{{ menu_page.menu-class }}"
                            {% endif %}>{{ menu_page.menu-title }}</a>
                </li>
            {% else %}
                <li class="dropdown">
                    {% assign submenu = menu_page.path
                                          | split: '.'
                                          | pop
                                          | join: '.' %}
                    <a class="dropdown-toggle {{ menu_page.menu-class }}"
                       href="#">{{ menu_page.menu-title }}</a>
                    <ul class="dropdown-menu">
                        {% include nav-menu.html path=submenu %}
                    </ul>
                </li>
            {% endif %}
        {% endif %}
    {% endif %}
{% endfor %}