Since micropython uses a utf-8 string encoding, the FAT long file names
(represented as UCS-2 on disk) should be encoded as UTF-8 on the API.
This fixes the problem that files on VFAT that have umlauts in their
file names are not shown correctly on the micropython side.
Signed-off-by: Matthias Blankertz <matthias@blankertz.org>