When in VariableParse, use the expand_cache if possible rather than looking
up data. Ultimately it would come from the same place but this short cuts
a heavily used code block for speed improvements.
(Bitbake rev: f682b8b83d21d576160bac8dc57c4c989b4dc555)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>