splash: Use splashfile instead of location->name

The splash image could be loaded from different sources (e.g. sf, mmc)
with different formats (e.g. raw, file-system). These sources are
structured by a board dependent object 'splash_location'. To decide
where is the splash image loaded, following environment variables are
used to select the splash source and file:
- 'splashsource' is used to select the splash source by setting its
  value to specified name of splash location.
- 'splashfile' specify the name of splash image file

But, when loads the splash image from FIT, the name of splash image
within FIT is specified by splash location name. Due to the splash
location name is already used for the splash source, its name may
conflicts with the name of splash image.

To solve the conflict, the environment variable 'splashfile' is used
to specify the splash image in FIT, and keeps the splash location
name for the splash source.

Signed-off-by: Leo Ruan <tingquan.ruan@cn.bosch.com>
Signed-off-by: Mark Jonas <mark.jonas@de.bosch.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Tomas Melin <tomas.melin@vaisala.com>