checkpolicy: rework host installation

checkpolicy has a pretty peculiar interpretation of DESTDIR and PREFIX.
PREFIX simply defaults to $(DESTDIR)/usr, and is used in the rest of
the build system. DESTDIR isn't used any further.

For the host installation, we don't want the usr part, so set PREFIX
instead of DESTDIR.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>