Makefile: Remove 3rd-party repos in make clean

The new target can be called directly:

    gmake clean-3rd-party-repos

Signed-off-by: Gerhard Lazu <glazu@pivotal.io>

Makefile

@@ -66,11 +66,16 @@ tag: $(abspath .)+tag $(READY_DEPS:%=$(DEPS_DIR)/%+tag)
 	git tag $(TAG) && \
 	echo
 
-clean:: clean-subrepos
+clean:: clean-subrepos clean-3rd-party-repos
 
 clean-subrepos: $(READY_DEPS:%=$(DEPS_DIR)/%+clean)
 	@:
 
+THIRD_PARTY_DEPS_DIRS = $(filter-out $(patsubst %,$(DEPS_DIR)/%,$(READY_DEPS)),$(wildcard $(DEPS_DIR)/*))
+
+clean-3rd-party-repos:
+	$(verbose) rm -rf $(THIRD_PARTY_DEPS_DIRS)
+
 %+clean:
 	-$(exec_verbose) $(MAKE) -C $* clean