splash: Introduce default_splash_locations

This change introduces default_splash_locations which
simplifies splash recovery from the first partition of
USB/MMC/SATA drive.

Given usual mapping of the first partition of external media for
basic boot stuff like uImage/zImage, .dtb etc it looks quite
obvious option to put there splash.bmp as well.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Simon Glass <sjg@chromium.org>
Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Signed-off-by: Anatolij Gustschin <agust@denx.de>