Fall back to stretch if d-i cannot be found in s-p-u (Closes: #902226).

Building from s-p-u is customary when preparing the d-i-n-i upload to
s-p-u, but a rebuild within stretch would failed after the point
release. Let's fix this by building against s-p-u by default (for the
regular upload), falling back to stretch if there's no matching version
in s-p-u.