Copter: booster output respects safety switch

[rmackay9]

Copter-3.6 (and higher) support a "booster" motor for multicopters (blog). These are configured by setting SERVOx_FUNCTION to 81 (boost_throttle) and then setting MOT_BOOST_SCALE to a postive number from 0 to 1.

This PR enables the output to the booster to reflect the safety switch instead of enabling it along with other "auxiliary" outputs (used for gimbals, etc).

This has been tested on an mRobotics Pixhawk1 under NuttX:

• connected a servo tester to read the output from MAIN OUT 5 of the pixhawk
• set MOT_BOOST_SCALE = 0.5, SERVO5_FUNCTION = 81
• rebooted pixhawk
• with safety switch unpressed (disengaged?), no PWM pulses were sent
• with safety switch pressed, PWM values were output (same value as held in SERVO5_MIN)

This item was reported in this Copter-3.6 beta testing thread.

P.S. I'm not totally overjoyed with the complexity of implementing the get_motor_mask() in the AP_MotorsMulticopter class and then calling it from all the children. I considered two other alternatives before implementing it this way:

• modify each child class's get_motor_mask() function to include the boost channel (overall less code actually).
• add a get_motor_mask_multicopter (or similar) and force the children to override this method instead of get_motor_mask (this would remove the need for the children to call the parent).

[rmackay9]

@tridge has quickly reviewed this and gave his OK. I'll merge on Monday but if any other opinions are also greatly appreciated.

[OXINARF]

LGTM

[rmackay9]

great, thanks for the reviews, merged!