[OpenPower-Firmware] Enforcing Developer Certificate of Origin for op-build

[Stewart Smith]

As per
https://github.com/open-power/op-build/blob/master/CONTRIBUTING.md#developer-certificate-of-origin
we require real names and "Signed-off-by" for commits to op-build.

I have been gradually trying to get people to do this over the last few
months.

As of when I make the op-build v1.19 tag (in about a week), commits to
be merged *MUST* follow these guidelines.

Notably, the current automated pull request generator
"hostboot at us.ibm.com" WILL NEED TO BE CHANGED, or code WILL NOT be
merged.

If doing things through the GitHub UI that produce commits, please note
that email addresses of '@users.noreply.github.com' are *NOT* okay.

We need this in order to have auditability of what code goes into
op-build and that those submitting it clearly have the right to do so
and have acknowledged that.

-- 
Stewart Smith
OPAL Architect, IBM.