Support `Fat_KV_RO` (a read-only k/v version of the FAT filesystem).

This requires mirage/ocaml-fat#20 to be merged first.
The `KV_RO` filesystem in Unix now passes through to the underlying filesystem instead of calling `crunch`

This requires mirage/mirage-fs-unix to be installed.
Merge pull request #160 from avsm/master

Support `Fat_KV_RO` (a read-only k/v version of the FAT filesystem).