Move the calls to difference_update out a code level which improves efficiency
significantly.
Also further combine the outer loop for efficiency too.
These two changes remove a bottleneck from the performance charts.
(Bitbake rev: e28ec69356f1797de3e4e3fca0fef710bc4564de)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>