AP_Motors: add new mixer class "AP_Motors_PX4", that delegates any mixing to the native PX4 mixer
add new pseudo frame type "px4mix", that delegates all frame-dependent mixing to the PX4 native mixer
PX4 mixer configuration files for coax frames
AP_MotorsPX4: reverted to deprecated header names to allow compilation against older NuttX revisions
enabled support for UAVCAN
AP_MotorsPX4: added MIX_ parameters
AP_MotorsPX4: receive motor limit flags from PX4 mixer
Merge branch 'master' of https://github.com/diydrones/ardupilot into px4mix

Conflicts:
	mk/targets.mk
Merge branch 'master' of https://github.com/diydrones/ardupilot into uavcan
Merge branch 'master' of https://github.com/diydrones/ardupilot into px4mix
AP_MotorsPX4: added motor test
AP_Motors_PX4: add custom frame support
enable build of motor_test utility
AP_MotorsPX4: implemented PWM ESC calibration
AP_MotorsPX4: set disarmed output value
AP_MotorsPX4: configure PX4IO PWM output
Merge branch 'master' of https://github.com/diydrones/ardupilot into px4mix
Merge branch 'master' of https://github.com/diydrones/ardupilot into uavcan
mixer templates: add idle
Merge branch 'master' of https://github.com/diydrones/ardupilot into px4mix