[VOL-5285] - JJB Upgrade toward v5

makefiles/include.mk
--------------------
  o Define new lib var sandbox-root= pointing at the build directory.
  o Added a comment section at the bottom to document declared vars.

makefiles/virtualenv/requirements-txt.mk
----------------------------------------
  o Rework pip -r requirements.txt logic to be dependency driven.
  o pip install will be run when requirements.txt is modified.

makefiles/virtualenv/include.mk
-------------------------------
  o Populate venv-abs-path= with sandbox-root= to insulate against
    $(PWD) path changes.
  o Split logic into separate makefiles so include.mk remains small.
  o Rework dependencies to leverage the new requirements-txt.mk logic.

Misc
----
  o Rework help help-* targets to not display everything, all the time.
  o make help      : display venv (primary target).
  o make venv-help : display exteneded help, venv targets and flags.

[HOWTO: test]
  % git clone onf-make
  % cd onf-make
  % touch requirements.txt

  % make help | grep venv
  % make venv-help

  % make venv
  % make venv-requirements

  % touch requirements.txt
  % make venv-requirements

Change-Id: I9e12bc45c73a0e2bc90b1fd11c87ccffc7ebe57a
diff --git a/makefiles/include.mk b/makefiles/include.mk
index 1bea64e..e9021ba 100644
--- a/makefiles/include.mk
+++ b/makefiles/include.mk
@@ -29,6 +29,9 @@
 ## -----------------------------------------------------------------------
 ## [LOADER] Define path vars based on library include directory
 ## -----------------------------------------------------------------------
+sandbox-root := $(dir $(abspath $(firstword $(MAKEFILE_LIST))))
+sandbox-root := $(patsubst %/,%,$(sandbox-root))
+
 $(foreach makefile,$(lastword $(MAKEFILE_LIST)),\
   $(foreach makedir,$(abspath $(dir $(makefile))),\
     $(eval include $(makedir)/library-makefiles.mk)\
@@ -90,4 +93,15 @@
 
 $(if $(DEBUG),$(warning LEAVE))
 
+## -----------------------------------------------------------------------
+## [VARS]
+##   sandbox-root     Path to top level directory containing [mM]akefile
+##
+##   onf-mk-top       Path to makefiles/ containing onf-make/ and local/
+##   onf-mk-tmp       Limited scratch area for repo:onf-make use
+##
+## [DEPRECATE]
+##   ONF_MAKEDIR      Replace with onf-mk-top/ {onf-mk}
+##   MAKEDIR          Replace with onf-mk-top/ {local}
+## -----------------------------------------------------------------------
 # [EOF]