[OpenPower-Firmware] Dropping RHEL6 support

[Stewart Smith]

I'm going to merge this patch into skiboot shortly,
https://patchwork.ozlabs.org/patch/1051614/  - which stops our CI
running on RHEL6 hosts.

RHEL6 is way into super-extended support (or whatever RedHat calls it)
and the hoops we jump through to build on RHEL6 make it more of a hack
environment than something that's sane to support.

If somebody wants to send patches when things break, that's fine, but
I'm not going to prevent any code going into skiboot that breaks on
RHEL6.

In the not too distant future, I expect to do the same for op-build.

-- 
Stewart Smith
OPAL Architect, IBM.