checkpatch: add check for too short Kconfig descriptions

I've seen various new Kconfigs with rather unhelpful one liner
descriptions.  Add a Kconfig warning for a minimum length of the Kconfig
help section.

Right now I arbitarily chose 4. The exact value can be debated.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>