AP_Motors: add support for offsetting collective when landed #940

Job	Run time
build (stm32f7, chibios, 10)	3m 37s
build (stm32h7, chibios, 10)	3m 37s
build (fmuv2-plane, chibios, 10)	1m 49s
build (periph-build, chibios, 10)	4m 24s
build (iofirmware, chibios, 10)	1m 39s
build (CubeOrange-bootloader, chibios, 10)	51s
build (fmuv3-bootloader, chibios, 10)	48s
build (revo-bootloader, chibios, 10)	48s
build (stm32h7-debug, chibios, 10)	2m 2s
build (fmuv3, chibios, 10)	5m 25s
build (revo-mini, chibios, 10)	4m 23s
build (MatekF405-Wing, chibios, 10)	4m 53s
build (CubeOrange-ODID, chibios, 10)	2m 44s
build (CubeRedPrimary-bootloader, chibios, 10)	47s
build (configure-all, chibios, 10)	10m 43s
build (build-options-defaults-test, chibios, 10)	6m 45s
build (signing, chibios, 10)	2m 34s
build (CubeOrange-PPP, chibios, 10)	2m 9s
build (CubeRed-EKF2, chibios, 10)	2m 17s
build (SOHW, chibios, 10)	3m 26s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 10s
build (new-check, chibios, 10)	4m 9s
	1h 11m 0s

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