- px4_add_module now requires MAIN
 - px4_add_library doesn't automatically link

Signed-off-by: Nicolae Rosia <nicolae.rosia@gmail.com>

It makes more sense to set the optimization flags on a platform basis
instead of individually for each module. This allows for different
optimization options for SITL, NuttX, Snapdragon, etc.

Now the mixer is a C++ library that can be fairly easily bolted into an output driver to provide mixing services.

Teach the FMU driver how to use it as an example.  More testing is still required.