fat/fs: move ls to generic implementation

Add a generic implementation of 'ls' using opendir/readdir/closedir, and
replace fat's custom implementation.  Other filesystems should move to
the generic implementation after they add opendir/readdir/closedir
support.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Łukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>