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>
diff --git a/Makefile b/Makefile
index 2861b4d..b35a472 100644
--- a/Makefile
+++ b/Makefile
@@ -1473,8 +1473,10 @@
 u-boot-lzma.img: u-boot.bin.lzma FORCE
 	$(call if_changed,mkimage)
 
+fit_image := $(if $(CONFIG_SANDBOX_VPL),u-boot,u-boot-nodtb.bin)
+
 u-boot-dtb.img u-boot.img u-boot.kwb u-boot.pbl u-boot-ivt.img: \
-		$(if $(CONFIG_SPL_LOAD_FIT),u-boot-nodtb.bin \
+		$(if $(CONFIG_SPL_LOAD_FIT),$(fit_image) \
 			$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_SANDBOX),dts/dt.dtb) \
 		,$(UBOOT_BIN)) FORCE
 	$(call if_changed,mkimage)