treewide: Use NULL for script image name

Two callers of image_source_script specify an image name. However, both
use the deprecated @ syntax, indicating that they have not been updated
in a while. If CONFIG_FIT_SIGNATURE is enabled, we will reject such
names outright. Back in commit 152576a5 ("stm32mp: stm32prog: handle
U-Boot script in flashlayout alternate"), we even renamed one of the
nodes. Instead of hard-coding a script image name, just use the default
image.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>