AP_Motors: add support for offsetting collective when landed #941

Job	Run time
build (stm32f7, chibios, 10)	16m 22s
build (stm32h7, chibios, 10)	14m 46s
build (fmuv2-plane, chibios, 10)	7m 56s
build (periph-build, chibios, 10)	21m 38s
build (iofirmware, chibios, 10)	4m 54s
build (CubeOrange-bootloader, chibios, 10)	3m 18s
build (fmuv3-bootloader, chibios, 10)	3m 17s
build (revo-bootloader, chibios, 10)	3m 34s
build (stm32h7-debug, chibios, 10)	9m 3s
build (fmuv3, chibios, 10)	16m 16s
build (revo-mini, chibios, 10)	12m 31s
build (MatekF405-Wing, chibios, 10)	13m 24s
build (CubeOrange-ODID, chibios, 10)	10m 31s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 19s
build (configure-all, chibios, 10)	11m 29s
build (build-options-defaults-test, chibios, 10)	21m 39s
build (signing, chibios, 10)	11m 41s
build (CubeOrange-PPP, chibios, 10)	8m 56s
build (CubeOrange-SOHW, chibios, 10)	9m 17s
build (Pixhawk6X-PPPGW, chibios, 10)	5m 22s
	3h 29m 13s

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