build: Allow CONCURRENCY_MAKE_LEVEL to be set on cmdline
build: Introduce CONCURRENCY_MAKE_FACTOR

It is often useful to apply a factor to N_CPUs when calculating the best
-j option for make. eg. N_CPUs*2.5 is usally good if your sources sit on
an SSD.
Merge pull request #3623 from t-nelson/make_concurrency

Tweaks for setting make's -j option