here is code for a login/logout control (instead of just a logout control)
<xp:div styleClass="pull-right">
<ul class="nav">
<li>
<xp:link
escape="true"
text="Log out"
id="link1">
<xp:this.rendered><![CDATA[#{javascript:@UserName() != "Anonymous"}]]></xp:this.rendered>
<xp:this.value><![CDATA[#{javascript:var contextPath = facesContext.getExternalContext().getRequest().getContextPath();
var requestURI = facesContext.getExternalContext().getRequest().getRequestURI();
strLogoutURL = contextPath + "?logout&redirectto=" + contextPath;
return strLogoutURL}]]></xp:this.value></xp:link>
</li>
<li>
<xp:link
escape="true"
text="Login"
id="link6">
<xp:this.rendered><![CDATA[#{javascript:@UserName() == "Anonymous"}]]></xp:this.rendered>
<xp:this.value><![CDATA[#{javascript:var contextPath = facesContext.getExternalContext().getRequest().getContextPath();
var requestURI = facesContext.getExternalContext().getRequest().getRequestURI();
strLoginURL = contextPath + "?login&redirectto=" + requestURI;
return strLoginURL}]]></xp:this.value></xp:link>
</li>
</ul>
</xp:div>