[OpenPower-Firmware] Deprecating RHEL6 as supported build env

[Stewart Smith]

After op-build v1.18, I have no intention of blocking anything going in if
there's an issue with builds done on RHEL6.

Currently, you can't build witherspoon-redbud on RHEL6 as the version of
patch is too old to understand file renames.

The overhead to actively maintain things building on RHEL6 is now too
high. Patches fixing any breakages will be judged on their own merit.

(this has been a long time coming, so shouldn't come as a too big
surprise).

-- 
Stewart Smith
OPAL Architect, IBM.