fs: add fat filesystem support ...

This code is based on:

http://elm-chan.org/fsw/ff/00index_e.html

FatFs Generic FAT File System Module

This patch offers a read/write implementation for barebox. The code
does not exaclty match barebox coding style, but works nicely and
should be ready to give it a try.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>