file_system/fat: Rework driver for fs_shell support

* Introduce fat_shell for build system fat manipulation
* Will theoretically let us do away with mtools when we
have another internal tool for partition manipulation

Change-Id: I661be556e79009842f157a9402c8f85da85d6336
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3556
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>