menuconfig: add support to show hidden options which have prompts

Usage:
  Press <Z> to show all config symbols which have prompts.

Quote Tim Bird:

| I've been bitten by this numerous times.  I most often
| use ftrace on ARM, but when I go back to x86, I almost
| always go through a sequence of searching for the
| function graph tracer in the menus, then realizing it's
| completely missing until I disable CC_OPTIMIZE_FOR_SIZE.
|
| Is there any way to have the menu item appear, but be
| unsettable unless the SIZE option is disabled?  I'm
| not a Kconfig guru...

I myself found this useful too. For example, I need to test
ftrace/tracing and want to be sure all the tracing features are
enabled, so I  enter the "Tracers" menu, and press <Z> to
see if there is any config hidden.

I also noticed gconfig and xconfig have a button "Show all options",
but that's a bit too much, and I think normally what we are not
interested in those configs which have no prompt thus can't be
changed by users.

Exmaple:

      --- Tracers
      -*-   Kernel Function Tracer
      - -     Kernel Function Graph Tracer
      [*]   Interrupts-off Latency Tracer
      - -   Preemption-off Latency Tracer
      [*]   Sysprof Tracer

Here you can see 2 tracers are not selectable, and then can find
out how to make them selectable.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/scripts/kconfig/lxdialog/menubox.c b/scripts/kconfig/lxdialog/menubox.c
index fa9d633..1d60473 100644
--- a/scripts/kconfig/lxdialog/menubox.c
+++ b/scripts/kconfig/lxdialog/menubox.c
@@ -383,6 +383,10 @@
 		case 'n':
 		case 'm':
 		case '/':
+		case 'h':
+		case '?':
+		case 'z':
+		case '\n':
 			/* save scroll info */
 			*s_scroll = scroll;
 			delwin(menu);
@@ -390,8 +394,10 @@
 			item_set(scroll + choice);
 			item_set_selected(1);
 			switch (key) {
+			case 'h':
+			case '?':
+				return 2;
 			case 's':
-				return 3;
 			case 'y':
 				return 3;
 			case 'n':
@@ -402,18 +408,12 @@
 				return 6;
 			case '/':
 				return 7;
+			case 'z':
+				return 8;
+			case '\n':
+				return button;
 			}
 			return 0;
-		case 'h':
-		case '?':
-			button = 2;
-		case '\n':
-			*s_scroll = scroll;
-			delwin(menu);
-			delwin(dialog);
-			item_set(scroll + choice);
-			item_set_selected(1);
-			return button;
 		case 'e':
 		case 'x':
 			key = KEY_ESC;