SITL: slung payload minor enhancements #32911

Job	Run time
build (chibios, Durandal)	19m 11s
build (chibios, MatekF405)	17m 2s
build (chibios, KakuteF7)	15m 52s
build (chibios, MatekH743-bdshot)	18m 57s
build (chibios, Pixhawk1-1M)	17m 43s
build (chibios, MatekF405-CAN)	4m 24s
build (chibios, DrotekP3Pro)	3m 53s
build (chibios, Hitec-Airspeed)	5m 42s
build (chibios, f103-GPS)	5m 23s
build (disco, armhf)	15m 32s
	2h 3m 39s

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