bitbake: parse/ast: Optimise data finalisation

bitbake: 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.

(Bitbake rev: b56918c7ef7913e84356c69ee9b269844a446728)

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