Makefile, dist/prepare: allow $OBJ to be overridden cleanly

If $OBJ is set – whether in environment or in `make` command line –
treat it as a temporary override, and do not touch the $HOSTOBJ symlink.
Makefile: remove knowledge about $OBJ structure

Let dist/prepare be the only place that knows about it.

(Yeah, yeah, I know about ~/.environ, I'll let it get away with that.)