fs: Fix issue with finding files in FAT32 partition

This patch reverts the below commits which were actually revert
to upstream patches. The earlier reverts were peformed for fixing
the FAT16 issue in u-boot. But, with those previous reverts,
the actual and most used FAT32 was broken. The issue is that
u-boot is not able to find all files present in partition, it can
only be able to find few files in partition.
So, this patch workarounds this by reverting the below
commits.

Revert "fat/fs: convert to directory iterators"
(sha1: 9681876)
Revert "fat/fs: move ls to generic implementation"
(sha1: 57bade5)
Revert "fs/fat: fix case for FAT shortnames"
(sha1: f975d9c)

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>