sandbox: Support obtaining the next phase from an image

sandbox: Support obtaining the next phase from an image

At present sandbox runs the next phase from discrete executables, so for
example u-boot-tpl runs u-boot-vpl to get to the next phase.

In some cases the phases are all built into a single firmware image, as is
done for real boards. Add support for this to sandbox.

Make it higher priority so that it takes precedence over the existing
method.

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