compulab: splash: support loading splash from sf

Add support for loading splash from sf.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
[grinberg@compulab.co.il: staticize the sf global variable]
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>