[Skiboot] OPAL console cleanups

[Oliver O'Halloran]

Patches 1-6 are misc cleanups of core/console.c

Patches 7-14 untangle the OPAL and skiboot consoles by removing the flush
operation from the skiboot con_ops structure and reworking the console
init path so the platform init code doesn't need to awkwardly step around
the dummy console forcing.

Hopefully these will make the console handling inside of skiboot a little
more approachable. In the future I'd like to further rework parts of it so
that most of the paramter validation for the OPAL_CONSOLE_* calls can be
handled in generic code rather than being repeated inside each OPAL console
backend, but this series is mostly cleanups to prepare for that.