sstate.bbclass: get the number of threads from BB_NUMBER_THREADS

sstate.bbclass: get the number of threads from BB_NUMBER_THREADS

- bitbake BB_NUMBER_THREADS uses cpu_count from oe utils that uses
the python os.sched_getaffinity and it is more acurrate.

 grep -nH ^BB_NUMBER_THREADS meta/conf/bitbake.conf
 meta/conf/bitbake.conf:806:BB_NUMBER_THREADS ?= "${@oe.utils.cpu_count()}"

- multiprocessing.cpu_count() returns the number of CPUs on the host,
not the number of usable CPUs on the host. If the user is using
scheduler affinity then the number of usable CPUs may be less,
so when determining how many cores we can use check the affinity instead.

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>