pkg-perl: add per package upgrade target

If PKG_DISTNAME is defined, an addition pkg-upgrade target is created
that will run scancpan with the right parameters for re-generating the
package.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
[Arnout:
  - make it really private to pkg-perl;
  - factor away the host/target condition, instead use $(4) directly.
]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>