docs: Add USB install method to user guide

Update the ONIE user guide to include a section on how to install a
NOS using a USB thumb drive.
docs: Document ONIE CLI commands

Add a section to the documentation describing the handful of ONIE CLI
commands.

- onie-discovery-start
- onie-discovery-stop
- onie-nos-install
- onie-self-update
- onie-support
- onie-sysinfo

Mark these older commands as deprecated:

- install_url
- update_url
- support
docs: clarify machine name consistency

Made it clear that the ONIE machine name remains the same irrespective
of what ONIE mode you are in.
docs: details for developers contributing patches

Added a few more guidelines for developers when submitting patches to
the project.
Add copyright notices to all files

To help the Open Compute Project protect the openness of the ONIE
project, proper copyright notices need to be in place for all the
applicable files.

This patch adds a copyright notice to most files based upon the
authors of the files as reported by 'git log'.  The format of the
notice is:

  #  Copyright (C) 2014 ONIE Developer <onie_helper@onie.org>
  #
  #  SPDX-License-Identifier:     GPL-2.0

The SPDX-License-Identifier is a short hand way to reference the
complete text of the GPLv2 license, which ONIE has used from the
beginning.  See the COPYING file for more on that.

  http://spdx.org/licenses/GPL-2.0
trivial: fix up Makefile copyright

Had a duplicate line in there...