fs: fat: add rename

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>