policycoreutils: rework host installation

policycoreutils has a pretty peculiar interpretation of DESTDIR and
PREFIX.  PREFIX is not consistently used: some installation paths and
include paths are forced to $(DESTDIR)/usr/... . In other cases,
PREFIX is indeed used. PREFIX defaults to $(DESTDIR)/usr

Try to be a little bit more correct by passing both DESTDIR and PREFIX,
both set to $(HOST_DIR). This is not a complete fix: some things are
still installed in $(HOST_DIR)/usr - but nothing we care about (just
manpages, systemd services, ...). More importantly, however, it still
looks for e.g. D-Bus in $(DESTDIR)/usr/include/dbus-1.0.

Still, it's better than nothing.

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