VOL-4836: VOLTHA release prep.

Makefile
--------
  o Update copyright notice.
  o include a few library makefiles for displaying help, etc.
  o Added target sterile to make clean + remove venv.
  o Replace inlined rm -fr, rm -f with make builtin $(RM) && $(RM) -r

tox.ini
-------
  o Add more interpreter versions to envlist so python 3.10.x can be used.

Change-Id: Ideb7e30ff703dff18b707226a4821455f5b1f261
diff --git a/makefiles/help/variables.mk b/makefiles/help/variables.mk
new file mode 100644
index 0000000..a50b233
--- /dev/null
+++ b/makefiles/help/variables.mk
@@ -0,0 +1,36 @@
+# -*- makefile -*-
+# -----------------------------------------------------------------------
+# Copyright 2017-2022 Open Networking Foundation
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# -----------------------------------------------------------------------
+
+# Include variables.mk after library makefiles have been included
+
+ifdef VERBOSE
+  help :: help-variables
+else
+  help ::
+	@echo
+	@echo '[VARIABLES] - Conditional makefile behavior'
+	@echo '  see also: help-variables'
+endif
+
+help-variables:
+	@echo
+	@echo '[VARIABLES] - Conditional makefile behavior'
+	@echo '  NO_PATCHES=           Do not apply patches to the python virtualenv'
+	@echo '  NO_OTHER_REPO_DOCS=   No foreign repos, only apply target to local sources.'
+	@echo '  VERBOSE=              Display extended help'
+
+# [EOF]