[coreboot] gerrit authentication

[Patrick Georgi]

Hi,

To provide more options to contributors, it's now possible to login to
our code review system at http://review.coreboot.org using GitHub
credentials. On the login screen, simply pick 'GitHub OAuth2'.

If you already have an account, you can link your GitHub account to it
by logging in on gerrit, clicking 'Link Another Identity' on
http://review.coreboot.org/#/settings/web-identities, and selecting
GitHub OAuth2 (and acknowleding on GitHub that you want to associate
your account with coreboot code review).

Of course, OpenID still works, as does logging in using Google
Accounts through OAuth2.

Regards,
Patrick