Update help, add convenience target git-submodule.

Makefile
--------
  o Added convenience target git-submodule to help checkout
    external repository dependencies.
  o Update target help formatting in prep for display of
    repo:onf-make library target help.

[HOWTO: Test]
  o git clone ssh://gerrit.opencord.org:29418/voltha-docs.git
  o cd voltha-docs
  o git review -d Id0ae53f924f3cae6b2da1edc39c6aeb966cb6907
  o make help-targets-main
  o make help | grep Alias  => Alias for git-submodules
  o make git-submodules     => (or make init)

Change-Id: Id0ae53f924f3cae6b2da1edc39c6aeb966cb6907
2 files changed