fs: fat: add rename

The implementation roughly follows the POSIX specification for
rename() [1]. The ordering of operations attempting to minimize the chance
for data loss in unexpected circumstances.

The 'mv' command was implemented as a front end for the rename operation
as that is what most users are likely familiar with in terms of behavior.

The 'FAT_RENAME' Kconfig option was added to prevent code size increase on
size-oriented builds like SPL.

[1] https://pubs.opengroup.org/onlinepubs/9799919799/functions/rename.html

Signed-off-by: Gabriel Dalimonte <gabriel.dalimonte@gmail.com>

test/py/tests/test_fs/fstest_helpers.py

@@ -9,5 +9,7 @@ def assert_fs_integrity(fs_type, fs_img):
     try:
         if fs_type == 'ext4':
             check_call('fsck.ext4 -n -f %s' % fs_img, shell=True)
+        elif fs_type in ['fat12', 'fat16', 'fat32']:
+            check_call('fsck.fat -n %s' % fs_img, shell=True)
     except CalledProcessError:
         raise