[Skiboot] skiboot console flushing and scraping

[Oliver O'Halloran]

This is based on my earlier random-cleanups series and does two main things:

A) It reworks how the OPAL console is flushed so that the flushing thread
will parse the header at the start of each Skiboot log entry to determine
the log level. This is to fix a long standing bug where when one thread is
flushing the skiboot console and a second thread writes into the console the
log level of the second message is discarded and will always be flushed.

B) It adds a new OPAL call that the operating system can use to scrape error
messages from the skiboot console. It also adds an OPAL event to notify the
current kernel that there are new messages to be scraped.

Patches 1-5: add a test for the console flush operation
Patches 6-8: Rework how flushing works to fix A)
Patches 9-10: Add the console scraping OPAL call.

I'm not super sold on the parse_loghdr() implementation and the exact semantics
of the OPAL call, but bikeshedding^Wfeedback is welcome.