expo: Add a configuration editor

Add a new 'cedit' command which allows editing configuration using an
expo. The configuration items appear as menus on the display.

This is extremely basic, only supporting menus and not providing any way
to load or save the configuration.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Makefile b/boot/Makefile
index 28c4e55..f828f87 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -33,6 +33,7 @@
 obj-$(CONFIG_CMD_BOOTEFI_BOOTMGR) += bootmeth_efi_mgr.o
 obj-$(CONFIG_$(SPL_TPL_)EXPO) += bootflow_menu.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootflow_menu.o
+obj-$(CONFIG_$(SPL_TPL_)CEDIT) += cedit.o
 endif
 
 obj-$(CONFIG_$(SPL_TPL_)OF_LIBFDT) += image-fdt.o