[OpenPower-Firmware] moving op-build dev from master-next to master

[Stewart Smith]

Hi all,

Currently, all new development work goes into op-build master-next
branch before moving to master close to the time where we tag a release.

This is rather different to literally every other project that uses git.

It's time we change.

After 1.14 (due Jan 18th), unless there's any strong objections, we'll
enact the following:

- pull requests start going to master: it's where development happens.
- master-next branch is deleted
- a vendor doing a release should branch off the most recent tagged
  release.
- stable branches, like v1.10.y will continue as-is

-- 
Stewart Smith
OPAL Architect, IBM.