kconfig: fix randconfig for choice blocks

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>