AP_RPM: Demystifying RPM sensors #38026

Job	Run time
build-gcc-heli	6m 14s
build (base)	6m 13s
build (clang)	6m 42s
autotest-heli (sitltest-heli)	12m 1s
autotest (sitltest-copter-tests1a)	9m 18s
autotest (sitltest-copter-tests1b)	11m 7s
autotest (sitltest-copter-tests1c)	13m 11s
autotest (sitltest-copter-tests1d)	10m 36s
autotest (sitltest-copter-tests1e)	10m 41s
autotest (sitltest-copter-tests2a)	10m 53s
autotest (sitltest-copter-tests2b)	31m 11s
	2h 8m 7s

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