Makefile library merge with onf-make and voltha-lib-go

makefiles/
----------
  o Import standalone makefile directories to checkin smaller parts of a mega patch.
  o Build out directory hierarchy by topic and feature.

makefiles/lint/yaml/
--------------------
  o Pull in logic evolved in repo:voltha-lib-go.
  o Refactoring: support special snowflake targets by repository.
  o ci-management and voltha-lib-go use distinct yamllint.conf files.
  o Separate until there is time to review and consolidate.
  o Added deps for on-demand local install of yamllint.
    - requirements.txt could be used but every build would incur overhead.

Makefile
--------
  o Use latest syntax from repo:onf-make.
  o No need to set path variables, makefiles/include.mk can infer them.
  o Added a help target to explicitly display targets and suggestions.
  o Logic is bundled in libraries and use of 'make help' is not well known.

Change-Id: Id8eba6f914a7bfe9a31637901bd8fb78a387e0c2
diff --git a/makefiles/consts.mk b/makefiles/consts.mk
index 8961b3a..f1f717f 100644
--- a/makefiles/consts.mk
+++ b/makefiles/consts.mk
@@ -34,8 +34,11 @@
 HIDE           ?= @
 
 env-clean      ?= /usr/bin/env --ignore-environment
-xargs-n1       := xargs -0 -t -n1 --no-run-if-empty
-xargs-n1-clean := $(env-clean) $(xargs-n1)
+
+xargs-cmd       := xargs -0 -t --no-run-if-empty
+xargs-n1        := $(xargs-cmd) -n1
+xargs-n1-clean  := $(env-clean) $(xargs-n1)
+xargs-cmd-clean := $(env-clean) $(xargs-cmd)
 
 ## -----------------------------------------------------------------------
 ## Intent: NOP command for targets whose dependencies do all heavy lifting