fs: fat: extend for in-PBL support ...

The AT91 BootROM loads a boot.bin file from the first FAT partition
into SRAM, when booting from MMC. To avoid the need for two barebox
configurations for each of the bootloader stages, add PBL support
for reading from FAT. This way each stage need only have a different
PBL entry point.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

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>