parse/ast: Optimise data finalisation

The optimisation where only the data we're interested in was finalised
was good but it turns out we can do better. In the case where a
class-extension is to be targeted, we can skip the other targets.

This change does that and speeds up parsing at the bitbake-worker
execution time. Specifically, you can see an improvement in the speed
of bitbake X -n.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake-worker: Drop BBHASH variables

Iterating through and calling setVar on this number of variables has significant
overhead in the profiling data. By not setting this, we save 3,000 calls
to setVar which gives a noticeable improvement to the speed of task execution.

The BBHASH variables have since been replaced by accessing that data through
the siggen code and going forward, that is the preferred way work with it.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>