checkpatch: allow optional shorter config descriptions

This script is used by many other projects, and in some of them the
requirement of at least 4 line long description for all Kconfig items is
excessive.  This patch adds a command line option to control the required
minimum length.

Tested running this script over a patch including a two line config
description.  The script generated a warning when invoked as is, and did
not generate it when invoked with --min-conf-desc-length=2.

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>