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>

scripts/kconfig/mconf.c

@@ -67,13 +67,15 @@ static const char mconf_readme[] = N_(
 "             there is a delayed response which you may find annoying.\n"
 "\n"
 "   Also, the <TAB> and cursor keys will cycle between <Select>,\n"
-"   <Exit> and <Help>\n"
+"   <Exit> and <Help>.\n"
 "\n"
 "o  To get help with an item, use the cursor keys to highlight <Help>\n"
-"   and Press <ENTER>.\n"
+"   and press <ENTER>.\n"
 "\n"
 "   Shortcut: Press <H> or <?>.\n"
 "\n"
+"o  To show hidden options, press <Z>.\n"
+"\n"
 "\n"
 "Radiolists  (Choice lists)\n"
 "-----------\n"
@@ -272,6 +274,7 @@ static int indent;
 static struct menu *current_menu;
 static int child_count;
 static int single_menu_mode;
+static int show_all_options;
 
 static void conf(struct menu *menu);
 static void conf_choice(struct menu *menu);
@@ -346,8 +349,16 @@ static void build_conf(struct menu *menu)
 	int type, tmp, doint = 2;
 	tristate val;
 	char ch;
+	bool visible;
 
-	if (!menu_is_visible(menu))
+	/*
+	 * note: menu_is_visible() has side effect that it will
+	 * recalc the value of the symbol.
+	 */
+	visible = menu_is_visible(menu);
+	if (show_all_options && !menu_has_prompt(menu))
+		return;
+	else if (!show_all_options && !visible)
 		return;
 
 	sym = menu->sym;
@@ -606,6 +617,9 @@ static void conf(struct menu *menu)
 		case 7:
 			search_conf();
 			break;
+		case 8:
+			show_all_options = !show_all_options;
+			break;
 		}
 	}
 }