Ignore C library files when checking coding style

The C library source files embedded into the Trusted Firmware
tree are not required to comply to the Linux Coding Style.
Unfortunately, 'make checkpatch' does take them into account.

This patch modifies the Makefile so that the C library source
and header files are now ignored by 'make checkpatch'.

It also instructs the checkpatch.pl script to not treat the
presence of a 'Change-Id' line in the commit message as an error.

Change-Id: I38196202efe518bae3a57c2affe2ed7758c9f69c

Makefile

@@ -77,10 +77,12 @@ TRUSTED_BOARD_BOOT	:= 0
 AUTH_MOD		:= none
 
 # Checkpatch ignores
-CHECK_IGNORE		=	--ignore COMPLEX_MACRO
+CHECK_IGNORE		=	--ignore COMPLEX_MACRO --ignore GERRIT_CHANGE_ID
 
 CHECKPATCH_ARGS		=	--no-tree --no-signoff ${CHECK_IGNORE}
 CHECKCODE_ARGS		=	--no-patch --no-tree --no-signoff ${CHECK_IGNORE}
+# Do not check the coding style on C library files
+CHECK_PATHS		=	$(shell ls -I include -I lib) $(shell ls -I stdlib include) $(shell ls -I stdlib lib)
 
 ifeq (${V},0)
 	Q=@
@@ -361,7 +363,7 @@ checkcodebase:		locate-checkpatch
 
 checkpatch:		locate-checkpatch
 			@echo "  CHECKING STYLE"
-			@git format-patch --stdout ${BASE_COMMIT} | ${CHECKPATCH} ${CHECKPATCH_ARGS} - || true
+			@git log -p ${BASE_COMMIT}..HEAD -- ${CHECK_PATHS} | ${CHECKPATCH} ${CHECKPATCH_ARGS} - || true
 
 .PHONY: ${CRTTOOL}
 ${CRTTOOL}: