sandbox: Build a device tree file for sandbox

Add support for building a device tree for sandbox's CONFIG_OF_HOSTFILE
option to make it easier to use device tree with sandbox.

This adjusts the Makefile to build a u-boot.dtb file which can be passed
to sandbox U-Boot with:

   ./u-boot -d u-boot.dtb

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 25af43b..9374108 100644
--- a/Makefile
+++ b/Makefile
@@ -708,6 +708,7 @@
 ALL-$(CONFIG_SPL_FRAMEWORK) += u-boot.img
 ALL-$(CONFIG_TPL) += tpl/u-boot-tpl.bin
 ALL-$(CONFIG_OF_SEPARATE) += u-boot.dtb u-boot-dtb.bin
+ALL-$(CONFIG_OF_HOSTFILE) += u-boot.dtb
 ifneq ($(CONFIG_SPL_TARGET),)
 ALL-$(CONFIG_SPL) += $(CONFIG_SPL_TARGET:"%"=%)
 endif