tools: fix clean rule in tool_chain_rump

The clean rule is used to delete already built binaries as well as to
clean-up any left-overs from previous build attempts. If there was
no previous attempt just return true to prevent make from complaining.

Fixes #1245.

tool/tool_chain_rump

@@ -109,7 +109,9 @@ install: $(CONTRIB_DIR) $(BUILD_DIR)/bin/rump_server
 
 clean:
 	$(VERBOSE)rm -rf $(BUILD_DIR)
-	$(VERBOSE)cd $(CONTRIB_DIR) && make cleanrump
+	$(VERBOSE)cd $(CONTRIB_DIR) > /dev/null 2>&1 && \
+	          rm .compile.tag > /dev/null 2>&1 && \
+	          make cleanrump > /dev/null 2>&1 || true
 
 cleanall: clean
 	$(VERBOSE)rm -rf $(CONTRIB_DIR)