[oe] Merging multimachine.bbclass into the OE.dev core

[Richard Purdie]

Hi,

multimachine has been a kind of second class citizen for a long time
having been maintained in its own class.  We now have things like sdk
generation which implicitly depend on multimachine being used. I can't
think of any reason people wouldn't want to use it and it allows extra
flexibility without imposing any added limitations.

Is it time we merged it into the core classes and made it the default?

The one drawback is it will force an effective wipe tmp for anyone who
wasn't using it before.

Cheers,

Richard