NuttX integration merge

This patch set removes all NuttX parts from the PX4 Firmware tree. It has undergone extensive bench testing for weeks (using real sensors and HIL) and should be ready for prime-time.

It would still be good to have some cross testing, but I have already based all my feature branches off this. In particular this branch helps with:

- Having latest NuttX with all fixes that have been implemented in the meantime (printf(), NaN in printf(), USB AND ttyS0, to name a few prominent examples)
- SPI DMA supported (though not enabled yet, there is a branch here with it enabled: https://github.com/PX4/nuttx/network)

There are some differences to current master in what files need to be checked out and the default target has been renamed to avoid confusion about which build system (old or new) we're using.

To convert an existing repo, perform these steps:

cd Firmware
rm -rf nuttx
git clone git@github.com:PX4/NuttX.git
make archives
make
make upload px4fmu-v1_default