Ingo Molnar reported that 'make randconfig' was not covering
choice blocks properly, resulting in certain config options
being left out of randconfig testing altogether.

With the following patch we:
- properly randomize choice value for normal choice blocks
- properly randomize for multi choice blocks
- added several comments to explain what is going on

The root cause of the bug was that SYMBOL_VALID was set on the
symbol representing the choice block so clearing this did
the trick initially.
But testign revealed a few more issues that is now fixed.
Reported-by: Ingo Molnar <mingo@elte.hu>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>