Provide CheckPatch configuration

Provie a CheckPatch configuration file and mention it in the coding style
guidelines (this feature was introduced with CheckPatch version 0.32 ~11 years
ago).

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

.checkpatch.conf

@@ -0,0 +1,12 @@
+--max-line-length=100
+--strict
+
+# Not Linux therefore don't expect a Linux tree
+--no-tree
+
+# Ignore the following
+--ignore PREFER_KERNEL_TYPES
+--ignore EXECUTE_PERMISSIONS
+--ignore FILE_PATH_CHANGES
+--ignore NEW_TYPEDEFS
+--ignore SPDX_LICENSE_TAG

CODINGSTYLE.md

@@ -4,8 +4,10 @@ Coding guidelines for s390-tools
 For s390-tools the preferred language is C. We provide libraries, e.g.
 [`libutil`](libutil) that should be used by all tools if possible.
 
-The coding style is based on the Linux [kernel guidelines]. Therefore, use
-the [checkpatch] tool for verification before you submit a patch.
+The coding style is based on the Linux [kernel guidelines]. Therefore, use the
+[checkpatch] tool for verification before you submit a patch. s390-tools
+provides a CheckPatch configuration for this - see
+[`.checkpatch.conf`](.checkpatch.conf).
 
 [kernel guidelines]: https://www.kernel.org/doc/html/latest/process/coding-style.html
 [checkpatch]: https://github.com/torvalds/linux/blob/master/scripts/checkpatch.pl