ArduPlane:make heading lock upon gnd speed consistent #1679

Job	Run time
build (stm32f7, chibios, 10)	13m 11s
build (stm32h7, chibios, 10)	12m 26s
build (fmuv2-plane, chibios, 10)	5m 29s
build (periph-build, chibios, 10)	13m 2s
build (iofirmware, chibios, 10)	2m 23s
build (CubeOrange-bootloader, chibios, 10)	1m 7s
build (fmuv3-bootloader, chibios, 10)	1m 0s
build (revo-bootloader, chibios, 10)	1m 5s
build (stm32h7-debug, chibios, 10)	7m 26s
build (fmuv3, chibios, 10)	13m 59s
build (revo-mini, chibios, 10)	11m 1s
build (MatekF405-Wing, chibios, 10)	11m 29s
build (CubeOrange-ODID, chibios, 10)	8m 57s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 3s
build (configure-all, chibios, 10)	11m 42s
build (build-options-defaults-test, chibios, 10)	21m 22s
build (signing, chibios, 10)	8m 44s
build (CubeOrange-PPP, chibios, 10)	6m 39s
build (CubeRed-EKF2, chibios, 10)	6m 33s
build (SOHW, chibios, 10)	10m 40s
build (Pixhawk6X-PPPGW, chibios, 10)	2m 57s
build (new-check, chibios, 10)	13m 17s
	3h 5m 32s

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