The default replacements (i.e. available for all the email templates) are:
$replacements += array(
'FORUMNAME' => $mbname,
'FORUMNAMESHORT' => (!empty($modSettings['maillist_sitename']) ? $modSettings['maillist_sitename'] : $mbname),
'EMAILREGARDS' => (!empty($modSettings['maillist_sitename_regards']) ? $modSettings['maillist_sitename_regards'] : ''),
'FORUMURL' => $boardurl,
'SCRIPTURL' => $scripturl,
'THEMEURL' => $settings['theme_url'],
'IMAGESURL' => $settings['images_url'],
'DEFAULT_THEMEURL' => $settings['default_theme_url'],
'REGARDS' => replaceBasicActionUrl($txt['regards_team']),
);
So you can add for example:
{FORUMURL}
and it will be replaced with the board url.