Move checkpatch options in a configuration file

At the moment, the top Makefile specifies the options to pass to the
checkpatch script in order to check the coding style. The checkpatch
script also supports reading its options from a configuration file
rather than from the command line.

This patch makes use of this feature and moves the checkpatch options
out of the Makefile. This simplifies the Makefile and makes things
clearer.

This patch also adds some more checkpatch options:
  --showfile
  --ignore FILE_PATH_CHANGES
  --ignore AVOID_EXTERNS
  --ignore NEW_TYPEDEFS
  --ignore VOLATILE
The rationale behind each of these options has been documented
in the configuration file.

Change-Id: I423e1abe5670c0f57046cbf705f89a8463898676
diff --git a/Makefile b/Makefile
index 9d183d2..74bbe42 100644
--- a/Makefile
+++ b/Makefile
@@ -107,11 +107,7 @@
 # Checkpatch script options
 ################################################################################
 
-CHECK_IGNORE		:=	--ignore COMPLEX_MACRO \
-				--ignore GERRIT_CHANGE_ID \
-				--ignore GIT_COMMIT_ID
-CHECKPATCH_ARGS		:=	--no-tree --no-signoff ${CHECK_IGNORE}
-CHECKCODE_ARGS		:=	--no-patch --no-tree --no-signoff ${CHECK_IGNORE}
+CHECKCODE_ARGS		:=	--no-patch
 # Do not check the coding style on imported library files or documentation files
 INC_LIB_DIRS_TO_CHECK	:=	$(sort $(filter-out			\
 					include/lib/libfdt		\
@@ -571,7 +567,7 @@
 
 checkpatch:		locate-checkpatch
 	@echo "  CHECKING STYLE"
-	${Q}git log -p ${BASE_COMMIT}..HEAD -- ${CHECK_PATHS} | ${CHECKPATCH} ${CHECKPATCH_ARGS} - || true
+	${Q}git log -p ${BASE_COMMIT}..HEAD -- ${CHECK_PATHS} | ${CHECKPATCH} - || true
 
 certtool: ${CRTTOOL}