build: add parameter to limit parallelism

Enable setting parameter to limit number of parallel jobs
when building components.
Keep default as nproc for maximal parallelism

Signed-off-by: Sanjyot Vaidya <Sanjyot.Vaidya@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Change-Id: Id6209e6208586b9b21420427d40e2a41c3f61892