Update .checkpatch.conf

- Don't require a description of every config statement in the
Kconfig changes
- Don't complain about externs in .c files
- Don't complain about the use of the volatile keyword.  The kernel
may not want it, but we definitely need it.
- Disable checks that seem to be broken.

Change-Id: Ic419b81cd36852a91e887e610d4a04984ab5fbd7
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/16010
Tested-by: build bot (Jenkins)
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>

.checkpatch.conf

@@ -1,9 +1,6 @@
 # Not Linux, so don't expect a Linux tree.
 --no-tree
 
-# Require every Kconfig symbol to have at least two lines in their description.
---min-conf-desc-length=2
-
 # Ignore aspects we don't follow here.
 --ignore C99_COMMENTS
 --ignore GLOBAL_INITIALISERS
@@ -15,3 +12,15 @@
 --ignore PREFER_PRINTF
 --ignore SPLIT_STRING
 --ignore BLOCK_COMMENT_STYLE
+--ignore AVOID_EXTERNS
+--ignore VOLATILE
+--ignore CONFIG_DESCRIPTION
+
+# FILE_PATH_CHANGES seems to not be working correctly. It will
+# choke on added / deleted files even if the MAINTAINERS file
+# is touched.
+--ignore FILE_PATH_CHANGES
+
+# This one has a linux path hard coded, so it would choke on
+# some commits unnecessarily.
+--ignore EXECUTE_PERMISSIONS