fs: change fatFS initialization to match littleFS.

Mounting the disk in a global function can lead to an issue
where fatFS cannot be mounted due to it not being registered beforehand.
To solve this problem, fatFS is now initialized in POST_KERNEL,
just like the littleFS initialization function.
fatfs_init must be called before attempting to mounte the disk.

Signed-off-by: Sihyun Noh <awake_noh@naver.com>