The documentation claims that Zephyr supports external file system
implementations, and there's no reason not to do so. Rework the API
to allow this.
Note that the file system type cannot legally be an enum anymore,
since we need to support file system types that don't have an
identifier assigned in that enum. Rely on the implicit conversion of
enum values to int to preserve backwards compatibility.
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
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