Added support for splash screen positioning

This patch adds support splash image positioning by adding an
additional variable "splashpos" to the environment. Please see
README for details.

Signed-off-by: Matthias Weisser <matthias.weisser@graf-syteco.de>
Acked-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Signed-off-by: Anatolij Gustschin <agust@denx.de>