Summary: - Due to the recent changes of wchar_t, ls command always causes errors for the fat file system. - This commit fixes this issue by replacing wchar_t with uint16_t under fs/fat Impact: - None Testing: - Tested with spresense:wifi and stm32f4discovery:wifi Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>