create a search path for vendor.conf

Now, if the build option is not specified, we'll first check /etc and then /usr/share. This allows OEMs to drop files in a location not managed by ostree, and allows distributions to have a default immutable config if not overridden by the OEM. If the build option is passed, the search path is disabled and the value from the build option used instead, so anyone currently using the build option won't notice any change (but also won't benefit from the addition of the search path).

Fixes #105 (closed)

WIP status is just because I haven't tested it yet. I will spin up a VM tomorrow to be cautious and make sure it still works.

Note it depends on !84 (merged) to avoid merge conflict there.