package/refpolicy: allow providing user defined modules

Allow users to provide custom SELinux modules to be part of the final
policy. A new configuration variable is added, pointing to list of
directories containing the custom modules.

SELinux modules do require a metadata.xml file to be well integrated
in the refpolicy build. If this file isn't provided, it will be
automatically created.

For now, this option requires the extra modules to be directly into
the BR2_REFPOLICY_EXTRA_MODULES directory, and subfolders aren't
supported.  They may never be, as having subfolders could introduce
issues when two different modules have the same name (which isn't
supported by the refpolicy).

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