[VOL-5194] - Update repo to use library makefiles.

https://gerrit.opencord.org/plugins/gitiles/onf-make
----------------------------------------------------
This patch will begin replacing the makefiles/ directory (target and logic
duplication) with centrally maintained library makefiles.  repo:onf-make
has been added as a git-submodule.

Simplest patch given the amount of changes needed.
It will only alter where duplicate target logic is included from.

.gitmodules
makefiles/onf-lib
-----------------
  o Checkout repo:onf-make as a submodule of voltha-docs.

makefiles/local/include.mk
--------------------------
  o Add a directory for per-repository makefile targets and overrides.

config.mk
makefiles/config.mk
-------------------
  o Relocate config.mk to avoid cluttering the repository root.

Makefile
--------
 o Replace high level imports with a single library makefile include.

makefiles-orig/virtualenv.mk
----------------------------
  o Help target cleanups, edits will migrate into repo:onf-make
    prior to removal of the local virtualenv.mk makefile.

makefiles/include.mk
--------------------
  o Added new install.mk include used to load onf-lib and local makefiles.
  o Staging: define onf-mk-lib as makefiles-orig/ for now.
  o Legacy makefile cleanup still needed before makefiles/local can be used

howto/index.rst
howto/nop.rst
---------------
  o Cosmetic edit to force a build/license check.
  o Last recheck attempt reported "No Builds Executed".

Depends-On: I7c83bd48d26510e79fd32f8bb97887cace67b55f
Depends-On: If75417c45363ee5c6a514fe9ebd4414ba8cf0cf7
Depends-On: I637d58f3e983b6cb1b7f08d19f16da925a579668

Change-Id: I8d7e53da91e52642726f1c8b997952765cbc1c23
Signed-off-by: Jan Klare <jan.klare@bisdn.de>
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..b524402
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "makefiles/onf-lib"]
+	path = makefiles/onf-lib
+	url = https://github.com/opencord/onf-make.git