checkpatch: allow passing config directory

checkpatch: allow passing config directory

checkpatch.pl searches for .checkpatch.conf in $CWD, $HOME and
$CWD/.scripts.  Allow passing a single directory via CHECKPATCH_CONFIG_DIR
environment variable (empty value is ignored).  This allows to directly
use project configuration file for projects which vendored checkpatch.pl
(e.g.  LTP or u-boot).

Although it'd be more convenient for user to have --conf-dir option
(instead of using environment variable), code would get ugly because
options from the configuration file needs to be read before processing
command line options with Getopt::Long.

While at it, document directories and environment variable in -h help
and HTML doc.

Link: https://lore.kernel.org/20260421211408.383972-1-pvorel@suse.cz
Signed-off-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Perches <joe@perches.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>