AP_Motors: add support for offsetting collective when landed #888

Job	Run time
build (base)	2m 9s
build (clang)	2m 3s
autotest (sitltest-plane-tests1a)	14m 33s
autotest (sitltest-plane-tests1b)	22m 6s
autotest (sitltest-quadplane)	14m 43s
	55m 34s

Provide feedback