This adds a `default: true` option for all cases where OSTree
information is specified in schemas and allows for the information
to be picked up from the filesystem.
This is a safe operation because when building disk images there is
no known case where having two deployments makes sense. In the case
there ever were a case then the osname, ref, and serial options still
exist and can be used.
Co-authored-by: Luke Yang <luyang@redhat.com>
Co-authored-by: Michael Vogt <michael.vogt@gmail.com>
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