AUTO_OBJ: Disable in non-build targets.

There's no reason to create object directories for targets like 'installworld'
or 'distributeworld', and the others in this list. Specifying MK_AUTO_OBJ as a
make argument allows circumventing this if needed for some reason.

This fixes mergemaster creating a full object tree due to doing a 'make
installconfig' tree walk.

Reported by: Mark Millard
Sponsored by: Dell EMC Isilon