pkg-infra: add <pkg>-extract and <pkg>-patch targets to OVERRIDE support

The legal-info target (and possibly others as well) depends on
<pkg>-extract to make sure the license file is available.  However,
when <PKG>_OVERRIDE_SRCDIR is active, the <pkg>-extract target
doesn't exist.

To solve this, we add <pkg>-extract which depends on <pkg>-rsync.
While we're at it, we do the same for <pkg>-patch.  That avoids the
same problem in the future if something starts depending on
<pkg>-patch.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>