Add checkpatch targets to Makefile

+ added target for calling checkpatch on working and staging area
+ added documentation for targets in Readme

Signed-off-by: Markus S. Wamser <markus.wamser@mixed-mode.de>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/Makefile b/Makefile
index c3c2698..96a16e6 100644
--- a/Makefile
+++ b/Makefile
@@ -107,3 +107,12 @@
 	${q}rm -f cscope.*
 	${q}find $(PWD) -name "*.[chSs]" | grep -v "$(PWD)/out" > cscope.files
 	${q}cscope -b -q -k
+
+.PHONY: checkpatch checkpatch-staging checkpatch-working
+checkpatch: checkpatch-staging checkpatch-working
+
+checkpatch-working:
+	${q}./scripts/checkpatch.sh
+
+checkpatch-staging:
+	${q}./scripts/checkpatch.sh --cached