sandbox: Add a new sandbox_spl board

sandbox: Add a new sandbox_spl board

It is useful to be able to build SPL for sandbox. It provides additional
build coverage and allows SPL features to be tested in sandbox. However
it does not need worthwhile to always create an SPL build. It nearly
doubles the build time and the feature is (so far) seldom used.

So for now, create a separate build target for sandbox SPL. This allows
experimentation with this new feature without impacting existing workflows.

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