package/refpolicy: allow to provide a custom refpolicy

Add support for the user to provide a fully custom refpolicy. When
this is used, modules aren't disabled anymore and packages do not
select refpolicy available modules either. The custom refpolicy must
define the full policy explicitly, and must be a fork of the original
refpolicy, to have the same build system.

This is added to allow users to fully control an SELinux policy, by
providing a complete custom policy.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>