Don't add Home breadcrumb if page is <front>

Problem/Motivation

If the variable menutrails_breadcrumbs is set, and the configured frontpage is part of a menu used in menutrail, then there is no need to set the breadcrumb at all when drupal_is_front_page() is true. Otherwise, as it is now, the frontpage will have a breadcrumb entry for itself.

Proposed resolution

Test for drupal_is_front_page() in menutrails_get_breadcrumbs() and return empty trail if true.