Build subprojects with multiple threads

[PhilippvK]

Currently subprojects such as muriscvnn and CMSISNN are build with a hardcoded number of threads which is currently either 1, 4 or $(nproc). As we use MLonMCU on machines with 4, 8, 24 cores we should adapt the number of used threads to the mlif.num_threads setting to use our resources efficiently. As the global -j argument can not be passed to the Makefile, we need to add an additional command line setting which can be set by MLonMCU: i.e. -DNUM_THREADS=