fs: allow mounting filesystems at /

Previously, filesystems could not be mounted at '/' because
mount points were restricted to being at least 2 characters.

Since '/' corresponds to the standard POSIX root filesystem
location, reduce the minimum length of a mount point to
1 character.

With that, we can mount a POSIX root filesystem.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>