bitbake.conf: Limit the number of OpenMP threads

Limits the number of OpenMP threads to match BB_NUMBER_THREADS. This
prevents OpenMP (libgomp in particular) from falling back to using all
the available CPUs, which behaves poorly when attempting to limit build
usage, especially when attempting to build in a container.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>