Reflect the updated behavior of the filesystem example and added more examples on the usage on real boards.
Remove the remarks about mtd and MTD_0.
Co-Authored-By: benpicco <benpicco@googlemail.com>
File systems should be mounted via `vfs_default`, not manually by
the application.
Also, `vfs` gained the `format` sub-command, so no need to provide
it in the example application.
