ports modules: Don't leak AUTO_OBJ changes into the port builds.

This came about when r328489 made ports modules builds no longer use the
in-tree share/mk files, but didn't cleanup MAKEOBJDIR from the
environment.

This fixes "Variable OBJTOP is recursive".

Sponsored by:	Dell EMC

sys/conf/kern.post.mk

@@ -70,6 +70,7 @@ PORTSMODULESENV=\
 	-u CXX \
 	-u CPP \
 	-u MAKESYSPATH \
+	-u MAKEOBJDIR \
 	MAKEFLAGS="${MAKEFLAGS:M*:tW:S/^-m /-m_/g:S/ -m / -m_/g:tw:N-m_*}" \
 	SYSDIR=${SYSDIR} \
 	PATH=${PATH}:${LOCALBASE}/bin:${LOCALBASE}/sbin \