package/refpolicy: allow packages to select SELinux modules

Add support for packages to enable SELinux modules already supported by
the refpolicy, but not selected by default in its policy.

With this commit, packages will be able to do something like:

SYSTEMD_SELINUX_MODULES = systemd udev

to enable additional SELinux modules.

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