[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....

- by passing J=AUTO from toplevel make it will auto-detect the number of CPUs
- umber of processes can also be explicitly set with e.g. J=4
- calling make without a J variable is the same as J=1 (serial compilation)

e.g. ```make AIRCRAFT=Quad_LisaM_2 ap.compile J=AUTO```

The question still remains if we should enable this by default...