Support for the native PX4 mixer

As discussed in https://groups.google.com/forum/?nomobile=true#!topic/drones-discuss/wV10ZUyLumU , these changes add support for the native mixer of the PX4 platform. The implemntaion adds an additional pseudo frame configuration named "PX4MIX_FRAME", that can be built by make "px4-v?-px4mix". The actual frame type is selected by the new parameter MIX_LAYOUT. For most popular layouts there is no need to edit PX4 mixer files, but advanced setups allow to do so.

The native mixer currently supports output to PWM (via PX4IO) and to UAVCAN. Support for other outputs (e.g. I2C or PX4FMU PWM) should be very easy to add.

This PR depends on a fix for issue #1321, otherwise the PWM output signal is disturbed.