Move the MAKEOBJDIRPREFIX value guard to sys.mk and expand to MAKEOBJDIR.

This will ensure that the variable was not set as a make override, in
make.conf, src.conf or src-env.conf. It allows setting the value in
src-env.conf when using WITH_AUTO_OBJ since that case properly handles
changing .OBJDIR (except if MAKEOBJDIRPREFIX does not yet exist which is
being discussed to be changed).

This change allows setting a default MAKEOBJDIRPREFIX via local.sys.env.mk.

Sponsored by: EMC / Isilon Storage Division