bitbake: data: fix performance regression

bitbake: data: fix performance regression

BitBake commit 7c568132c54a21161de28907159f902462f1e2bb resulted in a
fairly serious performance regression during parsing, almost doubling
the time taken to do a full parse and almost certainly impacting
performance during building. The expandKeys function is called
frequently, and if we avoid using keys() and instead just use the normal
variable lookup mechanism, performance is restored.

(Bitbake rev: 034b392e9877309f15940b258fc2c16f16fb40b5)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>