NuttX integration merge

Don't zero out the reported airspeed when below 15km/h

Optimise the RC input fetch for <= 9ch transmitters; this eliminates one read per cycle  from IO in the common case.

Copy the correct number of bytes back for register read operations. Basic PX4IO comms are working now.

Add more performance counters; run test #1 faster.

Enable handling for short-packet reception on FMU using the line-idle interrupt from the UART.  Enable short packets at both ends.

Report CRC, read and protocol errors.

Free perf counters in ~PX4IO_serial

Fix abort behaviour on timeouts, now we don't wedge after the first one.

Be more aggressive en/disabling DMA in the UART, since ST say you should.