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>