builddeb: fix orig tarball version

The version string in orig tarball needs to be the same as
packageversion with bits after last "-" stripped, since that
is the debian revision.

This fixes the deb-pkg source package when KDEB_PKGVERSION is
set.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>