Minor makefile clenaups.

Replace rm with macro $(RM) so 'make clean' will not fail when invoked in a clean sandbox.
Replaced 3-command pipeline to obtain branch= with an awk one-liner.

Change-Id: I03fce47713372d4b5979541efeaa4942dea0e6b1
diff --git a/Makefile b/Makefile
index 04819c4..7386da3 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-# Copyright 2021-present Open Networking Foundation
+# Copyright 2021-2022 Open Networking Foundation (ONF) and the ONF Contributors
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-branch=`cat .gitreview | grep branch | cut -d '=' -f2`
+branch=`awk -F\= '/branch=/ {print $2}' .gitreview`
 
 help: # @HELP Print the command options
 	@echo
@@ -40,5 +40,5 @@
 	@COMPARISON_BRANCH=origin/$(branch) ./helm-repo-tools/chart_version_check.sh
 
 clean: # @HELP Removes all files downloaded to run the tests
-	rm -rf helm-repo-tools
-	rm tagcollisionreject.*
+	$(RM) -r helm-repo-tools
+	$(RM) tagcollisionreject.*