Allow passing version for dpkg-genchanges to dpkg-buildpackage for -meta

[Herton Ronaldo Krzesinski]

When doing stable updates, we sometimes have cases where we need to
include more than one changelog entry on meta packages, for example, we
do one release to -proposed, this doesn't end up in updates, another
release/abi bump happens, and we must include 2 changelog entries for
the packages that will go to updates.

Following this is the proposed changes for hardy and the rest of meta
packages (lucid and everything supported until precise/q). It changes
the Makefile so one can easily specify the previous version so we can
tune the changelog generation as needed. It's a simple change, one could
always specify the -v manually, but since we already have a Makefile
that does everything... why not.

-- 
[]'s
Herton