[build] parallel build for firmwares

Speed up firmware compilation a lot by using parallel build.
This would be especially nice for running the compile_all_test_targets tests....

Classis sequential compilation can be forced by specifying J=1 from the commandline, e.g. ```make AIRCRAFT=Quad_LisaM_2 ap.compile J=1```

Unfortunately the gcc version is still printed slightly out of order instead of at the very beginning...

Please test on OSX, seems to work fine on LInux so far...

The question still remains if we should enable this by default...