tests: introduce test-checkstyle-diff

Make test-checkstyle-diff is a new target which checks PEP8 compliance
only for changed files. This makes it faster to execute and also more
readable as most of the time, only changed files will fail.

Type: improvement
Signed-off-by: Klement Sekera <ksekera@cisco.com>
Change-Id: I71baca76ab3a21a7a3790617cbfb0d48aacbd9ec
diff --git a/Makefile b/Makefile
index a1226a7..78300e2 100644
--- a/Makefile
+++ b/Makefile
@@ -206,6 +206,7 @@
 	@echo " checkstyle           - check coding style"
 	@echo " checkstyle-commit    - check commit message format"
 	@echo " checkstyle-test      - check test framework coding style"
+	@echo " checkstyle-test-diff - check test framework coding style (only changed files)"
 	@echo " checkstyle-api       - check api for incompatible changes"
 	@echo " fixstyle             - fix coding style"
 	@echo " doxygen              - (re)generate documentation"
@@ -478,6 +479,10 @@
 test-checkstyle:
 	@make -C test checkstyle
 
+.PHONY: test-checkstyle-diff
+test-checkstyle-diff:
+	@make -C test checkstyle-diff
+
 .PHONY: test-refresh-deps
 test-refresh-deps:
 	@make -C test refresh-deps