sandbox: Add a build for VPL

Add an initial VPL build for sandbox. This includes the flow:

   TPL (with of-platdata) -> VPL -> SPL -> U-Boot

To run it:

   ./tpl/u-boot-tpl -D

The -D is needed to get the default device tree, which includes the serial
console info.

Add a Makefile check for OF_HOSTFILE which is the option that enables
devicetree control on sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/Kconfig b/arch/sandbox/Kconfig
index 477c519..5f55c7f 100644
--- a/arch/sandbox/Kconfig
+++ b/arch/sandbox/Kconfig
@@ -29,6 +29,14 @@
 	bool "Enable SPL for sandbox"
 	select SUPPORT_SPL
 
+config SANDBOX_TPL
+	bool "Enable TPL for sandbox"
+	select SUPPORT_TPL
+
+config SANDBOX_VPL
+	bool "Enable VPL for sandbox"
+	select SUPPORT_VPL
+
 config SYS_CONFIG_NAME
 	default "sandbox_spl" if SANDBOX_SPL
 	default "sandbox" if !SANDBOX_SPL