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/test/boot/bootflow.c b/test/boot/bootflow.c
index ea1a994..6eeb59a 100644
--- a/test/boot/bootflow.c
+++ b/test/boot/bootflow.c
@@ -892,7 +892,7 @@
 	 *
 	 * Check both menu items, since there are two bootflows
 	 */
-	ut_assertok(check_font(uts, scn, OBJ_PROMPT, font_size));
+	ut_assertok(check_font(uts, scn, OBJ_PROMPT1A, font_size));
 	ut_assertok(check_font(uts, scn, OBJ_POINTER, font_size));
 	for (i = 0; i < 2; i++) {
 		ut_assertok(check_font(uts, scn, ITEM_DESC + i, font_size));