sandbox: Add ELF file to VPL u-boot.img

At present sandbox builds package up u-boot.bin in the .img file. This
cannot actually be executed, since it is not an ELF file.

For sandbox_vpl we want to be able to run the full boot flow.

Adjust the build rule for sandbox_vpl to package the ELF file and
thereby allow full testing of the sandbox transition from SPL to U-Boot
proper.

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