Makefile: add cscope target

Adds a cscope target to build the cross-reference files that can be used
by vim users to navigate the code.

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
diff --git a/Makefile b/Makefile
index c186d91..d17a6e6 100644
--- a/Makefile
+++ b/Makefile
@@ -226,3 +226,10 @@
 	$(echo) '  INSTALL ${DESTDIR}/bin'
 	$(q)mkdir -p ${DESTDIR}/bin
 	$(q)cp -a $(out-dir)/xtest/xtest ${DESTDIR}/bin
+
+.PHONY: cscope
+cscope:
+	$(echo) '  CSCOPE  .'
+	${q}rm -f cscope.*
+	${q}find $(PWD) -name "*.[ch]" -o -name "*.cpp" | grep -v /package/ > cscope.files
+	${q}cscope -b -q -k