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>