sandbox: Only delete the executable if requested

sandbox: Only delete the executable if requested

At present sandbox removes its executable after failing to run it,
since there is no other way that it would get cleaned up.

However, this is actually only wanted if the image was created within
sandbox. For the case where the image was generated by the build system,
such as u-boot-spl, we don't want to delete it.

Handle the two code paths accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>