web/web_favicon/views/templates.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        <template id="layout" inherit_id="web.layout">
            <xpath expr="//link[@rel='shortcut icon']" position="replace">
                <link rel="icon" href="/web_favicon/favicon" />
            </xpath>
        </template>
    </data>
</openerp>