Suppress Move task operations when main board is used as expansion

[dc42]

When a main board is used in expansion mode we should suppress the operations performed by the Move task so as to improve efficiency and avoid unwanted effects such as idle detection occurring.

One option is to terminate the Move task, but note that (a) we may want the Move task in future, see #1122, and (b) we will need to be careful that nothing nasty happens if the step ISR tries to wake up the Move task.