fix somewhat common edge case in $$shadowed()

fix somewhat common edge case in $$shadowed()

if source and build dir are direct children of the common root and we
are shadowing the top-level source dir, there is of course no trailing
slash to match.

Change-Id: I8a34a6a72d16cb21d77d056e037235af9b32a008
Reviewed-by: Davide Pesavento <davidepesa@gmail.com>
Reviewed-by: Andreas Hartmetz <ahartmetz@gmail.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>