Replace white/black master/slave terms with alternatives. We're not
changing "master" when used in the context of GitHub branches. GitHub
advises they have a plan to help this transition. In the text body we
rever to the "master" branch as the "main" branch, but leave any urls or
code-block commands still using "master".
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>
David B. Kinder