Teach the PX4 build system how to handle pre-built libraries.
Use the new prebuilt-library support to wrap the ARM CMSIS DSP library, and update to the version shipped with CMSIS 3.0 r3p2
Fix .gitignore to avoid ignoring prebuilt libraries.

Also, generally clean-up the .gitignores and farm off separate versions for the NuttX/Apps directories to keep things tidy.