expo: Support building an expo from a description file

The only way to create an expo at present is by calling the functions to
create each object. It is useful to have more data-driven approach, where
the objects can be specified in a suitable file format and created from
that. This makes testing easier as well.

Add support for describing an expo in a devicetree node. This allows more
complex tests to be set up, as well as providing an easier format for
users. It also provides a better basis for the upcoming configuration
editor.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Makefile b/boot/Makefile
index f94c31d..28c4e55 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -50,7 +50,7 @@
 obj-$(CONFIG_SPL_LOAD_FIT) += common_fit.o
 endif
 
-obj-$(CONFIG_$(SPL_TPL_)EXPO) += expo.o scene.o scene_menu.o
+obj-$(CONFIG_$(SPL_TPL_)EXPO) += expo.o scene.o scene_menu.o expo_build.o
 
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE) += vbe.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE_REQUEST) += vbe_request.o