package/refpolicy: allow packages to provide their own SELinux modules

Allow packages to have an 'selinux' subfolder containing SELinux modules
(sources) to be synced and compiled within the refpolicy, if the package
is selected.

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