package/refpolicy: allow selecting additional modules

Allow users to select additional modules available in the refpolicy, to
be built in the binary policy. This will allow non-base modules to be
selected based on the user use-case and to select extra module
dependencies when providing out-of-tree modules.

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