Copter: Guard External Control with a define #3197

Job	Run time
build (base)	3m 5s
build (clang)	2m 52s
build-gcc-heli	3m 55s
autotest (sitltest-copter-tests1a)	18m 52s
autotest (sitltest-copter-tests1b)	20m 2s
autotest (sitltest-copter-tests1c)	19m 34s
autotest (sitltest-copter-tests1d)	17m 38s
autotest (sitltest-copter-tests1e)	15m 36s
autotest (sitltest-copter-tests2a)	16m 8s
autotest (sitltest-copter-tests2b)	26m 2s
autotest-heli (sitltest-heli)	17m 34s
	2h 41m 18s

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