package/refpolicy: test REFPOLICY_EXTRA_MODULES_DIR differently

package/refpolicy: test REFPOLICY_EXTRA_MODULES_DIR differently

REFPOLICY_EXTRA_MODULES_DIRS contains
$(PACKAGES_SELINUX_EXTRA_MODULES_DIRS) which is filled in by
package/pkg-generic.mk with the list of packages that have a selinux/
sub-directory. Due to how variable expansion works, if there is an
ifeq/ifneq test of REFPOLICY_EXTRA_MODULES_DIRS, it will only see the
value of REFPOLICY_EXTRA_MODULES_DIRS with the list of packages
*before* refpolicy in alphabetic ordering. This means that packages
after refpolicy in alphabetic ordering would not be taken into
account.

To fix this, we switch to an $(if ...) test, which allows the variable
to really be evaluated during the refpolicy build. This makes sures
the expansion is correct.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>