Add the possibility to pass options for checkpatch

It can be handy for example to add --strict option which can detect more
coding issues, even if not mandated by TF-A coding rules.
To use it:
 CHECKPATCH_OPTS="--strict" make checkpatch

Change-Id: I707e4cc2d1250b21f18ff16169b5f1e5ab03a7ed
Signed-off-by: Yann Gautier <yann.gautier@st.com>

Makefile

@@ -841,13 +841,18 @@ checkcodebase:		locate-checkpatch
 
 checkpatch:		locate-checkpatch
 	@echo "  CHECKING STYLE"
+	@if test -n "${CHECKPATCH_OPTS}"; then				\
+		echo "    with ${CHECKPATCH_OPTS} option(s)";		\
+	fi
 	${Q}COMMON_COMMIT=$$(git merge-base HEAD ${BASE_COMMIT});	\
 	for commit in `git rev-list $$COMMON_COMMIT..HEAD`; do		\
 		printf "\n[*] Checking style of '$$commit'\n\n";	\
 		git log --format=email "$$commit~..$$commit"		\
-			-- ${CHECK_PATHS} | ${CHECKPATCH} - || true;	\
+			-- ${CHECK_PATHS} |				\
+			${CHECKPATCH} ${CHECKPATCH_OPTS} - || true;	\
 		git diff --format=email "$$commit~..$$commit"		\
-			-- ${CHECK_PATHS} | ${CHECKPATCH} - || true;	\
+			-- ${CHECK_PATHS} |				\
+			${CHECKPATCH}  ${CHECKPATCH_OPTS} - || true;	\
 	done
 
 certtool: ${CRTTOOL}