kconfig: add merge_config.sh script

After noticing almost every distro has their own method of managing config
fragments, I went looking at some best practices, and wanted to try to
consolidate some of the different approaches so this fairly simple
infrastructure can be shared (and new distros/build systems don't have to
implement yet another config fragment merge script).

This script is most influenced by the Windriver tools used in the Yocto
Project, reusing some portions found there.

This script merges multiple config fragments, warning on any overridden
values.  It then sets any unspecified values to their default, then
finally checks to make sure no specified value was dropped due to
unsatisfied dependencies.

I'm sure this implementation won't work for everyone, and I expect it will
need to evolve to adapt for various use cases.  But I think its a
reasonable starting point.

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Greg Thelen <gthelen@google.com>
Cc: Reinhard Tartler <Reinhard.Tartler@informatik.uni-erlangen.de>
Cc: Dmitry Fink <Dmitry.Fink@palm.com>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Eric B Munson <ebmunson@us.ibm.com>
Cc: Bruce Ashfield <Bruce.Ashfield@windriver.com>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>