Remove MK_AUTO_OBJ from the build environment provided to each port in PORTS_MODULES.
This fixes a failure to resolve object file paths seen when buildkernel (which sets MK_AUTO_OBJ=yes) and installkernel (which sets MK_AUTO_OBJ=no) are run as separate steps. r329232 partially fixed this scenario by removing MAKEOBJDIR, but it seems AUTO_OBJ settings also need to be on the same page for these two steps.