VOL-4836: Interactive helm chart linting.

repo:voltha-docs job reported reactive post-commit linting errors.
Begin adding support for make target lint-helm for early detection.

Makefile
--------
  o Adding comments for readability.
  o Disable failing/unused targets.
  o Update help text.
  o Migrating some targets and deps from Makefile proper into makefiles/*.

makefiles/README.md
-------------------
  o Added a basic makefile readme with pointers to other web resources.

makefiles/lint/include.mk
makefiles/lint/helm/include.mk
-------------------------
  o Initial support for library makefile target: 'lint-helm'
  o Added common targets: clean, help, sterile and todo.
  o Target 'pre-commit added:
      - default target to support interactive use of lint-helm.
      - Cleanup needed before common target can use lint-helm as a default.
  o Interim answer: briding the gap between existing and portable logic.
  o legacy logic (USE_LEGACY=1 (default)):
      - Relies on .gitreview and hardcoding defaults (opencord/master).
      - git has builtin support for local/remote branch queries.
      - updates to support queries which simplify logic.
  o Interactive target lint-helm-deps
    - Configure helm charts per linting logic in gerrit/jenkins job.
    - Interactive target will invoke make target with USE_LEGACY=$(null).

Change-Id: I43616d16707a09da6f907f909c65504da9933a6f
diff --git a/makefiles/lint/include.mk b/makefiles/lint/include.mk
index f904217..d3975de 100644
--- a/makefiles/lint/include.mk
+++ b/makefiles/lint/include.mk
@@ -13,14 +13,25 @@
 # 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.
+#
+# SPDX-FileCopyrightText: 2022 Open Networking Foundation (ONF) and the ONF Contributors
+# SPDX-License-Identifier: Apache-2.0
 # -----------------------------------------------------------------------
 
-help::
-	@echo
-	@echo "[LINT]"
+# ------------------------------------------------
+# On-demand loading:
+#   - lint helm charts
+#   - on demand for now, errors and cleanup needed
+#   - Usage: make lint-helm USE_LEGACY=<blank>
+# ------------------------------------------------
+$(if $(findstring lint-helm,$(MAKECMDGOALS)),\
+  $(eval include $(MAKEDIR)/lint/helm/include.mk))
 
-include $(MAKEDIR)/lint/helm.mk
-
+# ------------------------------------------------------------
+## yaml checking
+##   o yamllint - check everything, all the time.
+##   o python   - dependency based checking
+# ------------------------------------------------------------
 ifdef YAML_FILES
   include $(MAKEDIR)/lint/yaml/python.mk
 else