bootstd: Support creating a boot menu

Create an expo to handle the boot menu. For now this is quite simple, with
just a header, some menu items and a pointer to show the current one.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Makefile b/boot/Makefile
index 0b30fcd..f990e66 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -30,6 +30,7 @@
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_SCRIPT) += bootmeth_script.o
 ifdef CONFIG_$(SPL_TPL_)BOOTSTD_FULL
 obj-$(CONFIG_$(SPL_TPL_)CMD_BOOTEFI_BOOTMGR) += bootmeth_efi_mgr.o
+obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootflow_menu.o
 endif
 
 obj-$(CONFIG_$(SPL_TPL_)OF_LIBFDT) += image-fdt.o