scripts: Separate Capstan and VM image building

This separates Capstan and VM image building into two scripts. This is
needed to be able to drop useless VM images like "osv-base" or
"osv-openjdk" that only make sense as Capstan base images, not
standalone VM images that people boot to.

Cc: Tzach Livyatan <tzach@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>