checkpatch: change format of --color argument to --color[=WHEN]

checkpatch: change format of --color argument to --color[=WHEN]

The boolean --color argument did not offer the ability to force
colourized output even if stdout is not a terminal.  Change the format
of the argument to the familiar --color[=WHEN] construct as seen in
common Linux utilities such as git, ls and dmesg, which allows the user
to specify whether to colourize output "always", "never", or "auto" when
the output is a terminal.  The default is "auto".

The old command-line uses of --color and --no-color are unchanged.

Link: http://lkml.kernel.org/r/efe43bdbad400f39ba691ae663044462493b0773.1496799721.git.joe@perches.com
Signed-off-by: John Brooks <john@fastquake.com>
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>