expo: Improve the visual appearance of the menu

The menu is currently quite basic. Make use of some recently added
features in expo, to:

- Show proper prompts
- Highlight the current item
- Centre text
- Use multi-line text instead of two independent lines
- Put a box around the items

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/bootflow_internal.h b/boot/bootflow_internal.h
index 38cf02a..4cdb696 100644
--- a/boot/bootflow_internal.h
+++ b/boot/bootflow_internal.h
@@ -14,7 +14,10 @@
 	START,
 
 	/* strings */
-	STR_PROMPT,
+	STR_PROMPT1A,
+	STR_PROMPT1B,
+	STR_PROMPT2,
+	STR_AUTOBOOT,
 	STR_MENU_TITLE,
 	STR_POINTER,
 
@@ -23,10 +26,14 @@
 
 	/* objects */
 	OBJ_U_BOOT_LOGO,
+	OBJ_BOX,
 	OBJ_MENU,
-	OBJ_PROMPT,
+	OBJ_PROMPT1A,
+	OBJ_PROMPT1B,
+	OBJ_PROMPT2,
 	OBJ_MENU_TITLE,
 	OBJ_POINTER,
+	OBJ_AUTOBOOT,
 
 	/* strings for menu items */
 	STR_LABEL = 100,