uboot-imx

splash_source: add support for filesystem formatted mmc

Add support for loading splash image from an SD card formatted with
a filesystem. Update boards to maintain original behavior where needed.

Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>