AP_DDS: External Control enable #41145

Job	Run time
build (base)	6m 31s
build (clang)	3m 40s
build-gcc-heli	3m 22s
autotest-heli (sitltest-heli)	10m 2s
autotest (sitltest-copter-tests1a)	7m 13s
autotest (sitltest-copter-tests1b)	9m 10s
autotest (sitltest-copter-tests1c)	11m 52s
autotest (sitltest-copter-tests1d)	8m 53s
autotest (sitltest-copter-tests1e)	9m 15s
autotest (sitltest-copter-tests2a)	8m 59s
autotest (sitltest-copter-tests2b)	28m 43s
	1h 47m 40s

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