base_path used for logo source when no default/custom logo is defined

Committed to HEAD, thanks.

I'am not sure this is quite a good idea. Adding drupal base path to every logo path can cause trouble : i have defined a file upload directory that is not in my drupal path (for exemple /var/www/files and for drupal /var/www/html/drupal/). So with the patch '$logo' is /drupal/var/www/files and it does'nt work.