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>

boot/bootflow_internal.h

@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Internal header file for bootflow
+ *
+ * Copyright 2022 Google LLC
+ * Written by Simon Glass <sjg@chromium.org>
+ */
+
+#ifndef __BOOTFLOW_INTERNAL_H
+#define __BOOTFLOW_INTERNAL_H
+
+/* expo IDs for elements of the bootflow menu */
+enum {
+	START,
+
+	/* strings */
+	STR_PROMPT,
+	STR_MENU_TITLE,
+	STR_POINTER,
+
+	/* scene */
+	MAIN,
+
+	/* objects */
+	OBJ_U_BOOT_LOGO,
+	OBJ_MENU,
+	OBJ_PROMPT,
+	OBJ_MENU_TITLE,
+	OBJ_POINTER,
+
+	/* strings for menu items */
+	STR_LABEL = 100,
+	STR_DESC = 200,
+	STR_KEY = 300,
+
+	/* menu items / components (bootflow number is added to these) */
+	ITEM = 400,
+	ITEM_LABEL = 500,
+	ITEM_DESC = 600,
+	ITEM_KEY = 700,
+	ITEM_PREVIEW = 800,
+
+	/* left margin for the main menu */
+	MARGIN_LEFT	 = 100,
+};
+
+#endif /* __BOOTFLOW_INTERNAL_H */