scripts: Add upload script for standalone images

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
scripts: Rename Capstan release upload script

Rename the script to "upload-capstan-release" to match the naming
convention of "upload-standalone-release".

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>