sandbox: Add an image for VPL

sandbox: Add an image for VPL

Use binman to build an image which includes all the U-Boot phases so that
a full VBE boot can take place with just that image.bin file. Attach the
image file to mmc2 so it can be loaded.

VBE is used to load images in two phases:

   - In VPL, VBE decides which SPL image to load
   - In SPL, VBE decides which U-Boot image to load

The latter should really be determined by VPL, since it does the full
signature verification on the selected configuration. However, we have
separate configurations for SPL and U-Boot proper, so for now we keep it
simple and have SPL do its own verification. This will need to be
tidied up later.

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