sandbox_vpl: Enable Universal Payload

Use the sandbox_vpl build to test UPL since it supports a real devicetree
in SPL. The sandbox_spl build uses OF_PLATDATA.

Enable writing the UPL handoff in SPL and reading it in U-Boot proper.
Provide a test to check that this handoff works.

Note that the test uses the standard devicetree rather than the test one,
since it is a lot smaller and fits in the existing bloblist.

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