Use buildconf/num_procs (RBM_NUM_PROCS) for firebox build

We have an option buildconf/num_procs (or the RBM_NUM_PROCS env variable) which we use to select the number of parallel jobs for builds. We use it in some components, but we currently don't use it for firefox.

According to https://firefox-source-docs.mozilla.org/setup/configuring_build_options.html we can set this for firefox with a line like mk_add_options MOZ_PARALLEL_BUILD=4 in the mozconfig file.

While changing this, maybe we can also change the default value of buildconf/num_procs (currently 4 if RBM_NUM_PROCS is not set) to be the number of cpus that can be seen in /proc/cpuinfo.