Makefile edits to improve library lint target support.

Makefile
--------
  o Replace inlined includes with single $(MAKEDIR)/include.mk
  o virtualenv install w/dependencies handled by makefiles/virtualenv.mk
  o Use '&&' for command separation to improve error handling.
  o Added more target documentation.
  o Re-home doc8/rst syntax checking into makefiles/lint/doc8.mk.

howto/code/linting.rst
----------------------
  o Document supported lint targets on docs.voltha.org.

howto/release/index.rst
howto/release/post-release/
howto/release/vcs/
---------------------------
  o Document more release related activity (bugfix branch creation)

makefiles/consts.mk
-------------------
  o Define and use more library macros.

config.mk
---------
  o Per-repository library makefile configuration, disable linting.

makefiles/virtualenv.mk
-----------------------
  o Library virtualenv install with dependences.
  o Begin using a consistent install directory.
  o Ugh: repository (voltha-docs) submodules each install a full venv.
  o Install dependency drive based on existence of activate script.
  o Lint targets updated to use activate script as a dependency.
  o Update activate macro to run:  set -u && activate && set +u

makefiles/patches/include.mk
----------------------------
  o Update to use virtualenv.mk defined macros.

makefiles/lint/doc8.mk
makefiles/lint/python/flake8.mk
makefiles/lint/json.mk
makefiles/lint/python/pylint.mk
makefiles/lint/robot.mk
makefiles/lint/shell.mk
makefiles/lint/yaml.mk
-------------------------------
  o All library linting defines consistent targets and logic.
  o Support two library targets: lint-all and lint-modified.
  o Default to checking all available sources.
  o Linting can be disabled per-repository via {project}/config.mk

Change-Id: I7b7b5e523c867183d44244a0da21cfa3584e9a50
diff --git a/makefiles/lint/yaml.mk b/makefiles/lint/yaml.mk
index c875686..566ac18 100644
--- a/makefiles/lint/yaml.mk
+++ b/makefiles/lint/yaml.mk
@@ -1,8 +1,8 @@
 # -*- makefile -*-
 # -----------------------------------------------------------------------
-# Copyright 2017-2023 Open Networking Foundation
+# Copyright 2017-2023 Open Networking Foundation (ONF) and the ONF Contributors
 #
-# Licensed under the Apache License, Version 2.0 (the "License");
+# 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
 #
@@ -15,18 +15,68 @@
 # limitations under the License.
 # -----------------------------------------------------------------------
 
-YAML_FILES ?= $(error YAML_FILES= is required)
+$(if $(DEBUG),$(warning ENTER))
 
-.PHONY: lint-yaml
+##-------------------##
+##---]  GLOBALS  [---##
+##-------------------##
+.PHONY: lint-yaml lint-yaml-all lint-yaml-modified
 
-lint : lint-yaml
+have-yaml-files := $(if $(strip $(YAML_FILES)),true)
+YAML_FILES      ?= $(error YAML_FILES= is required)
 
-lint-yaml: vst_venv
-	source ./$</bin/activate \
-	    ; set -u \
-	    ; yamllint -s $(YAML_FILES)
+## -----------------------------------------------------------------------
+## Intent: Use the yaml command to perform syntax checking.
+##   o If UNSTABLE=1 syntax check all sources
+##   o else only check sources modified by the developer.
+## Usage:
+##   % make lint UNSTABLE=1
+##   % make lint-yaml-all
+## -----------------------------------------------------------------------
+ifndef NO-LINT-YAML
+  lint-yaml-mode := $(if $(have-yaml-files),modified,all)
+  lint : lint-yaml-$(lint-yaml-mode)
+endif# NO-LINT-YAML
 
+## -----------------------------------------------------------------------
+## Intent: exhaustive yaml syntax checking
+## -----------------------------------------------------------------------
+lint-yaml-all:
+	$(HIDE)$(MAKE) --no-print-directory lint-yaml-install
+
+	find . \( -iname '*.yaml' -o -iname '*.yml' \) -print0 \
+	    | $(xargs-n1-clean) yamllint --strict
+
+## -----------------------------------------------------------------------
+## Intent: check deps for format and python3 cleanliness
+## Note:
+##   yaml --py3k option no longer supported
+## -----------------------------------------------------------------------
+lint-yaml-modified:
+	$(HIDE)$(MAKE) --no-print-directory lint-yaml-install
+	yamllint -s $(YAML_FILES)
+
+## -----------------------------------------------------------------------
+## Intent:
+## -----------------------------------------------------------------------
+lint-yaml-install:
+	@echo
+	@echo "** -----------------------------------------------------------------------"
+	@echo "** yaml syntax checking"
+	@echo "** -----------------------------------------------------------------------"
+	yamllint --version
+	@echo
+
+## -----------------------------------------------------------------------
+## Intent: Display command usage
+## -----------------------------------------------------------------------
 help::
-	@echo "  lint-yaml            Syntax check yaml source using yamllint"
+	@echo '  lint-yaml          Syntax check python using the yaml command'
+  ifdef VERBOSE
+	@echo '  lint-yaml-all       yaml checking: exhaustive'
+	@echo '  lint-yaml-modified  yaml checking: only modified'
+  endif
+
+$(if $(DEBUG),$(warning LEAVE))
 
 # [EOF]