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/board/sandbox/MAINTAINERS b/board/sandbox/MAINTAINERS
index d32561c..9f5073b 100644
--- a/board/sandbox/MAINTAINERS
+++ b/board/sandbox/MAINTAINERS
@@ -33,3 +33,10 @@
 F:	board/sandbox/
 F:	include/configs/sandbox.h
 F:	configs/sandbox_flattree_defconfig
+
+SANDBOX VPL BOARD
+M:	Simon Glass <sjg@chromium.org>
+S:	Maintained
+F:	board/sandbox/
+F:	include/configs/sandbox_spl.h
+F:	configs/sandbox_vpl_defconfig