splash: get devpart from environment variable

splash: get devpart from environment variable

By default several types of splash locations are supported and the
user can select one of them through environment var (splashsource).

However the devpart is still hardcoded and we cannot change it from
the environment.

This patch add the support of "splashdevpart" which allow the user to
set the devpart though this environment variable.

Example: image located in splashscreen partition (MMC as raw)
```
splashsource=mmc_raw
splashdevpart=0#splashscreen
```

Signed-off-by: Julien Masson <jmasson@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>