Remove MK_AUTO_OBJ from env passed to 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 the AUTO_OBJ setting also needs to be on the same
page for the build and install steps.

Reviewed by:	bdrewery
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D14143

sys/conf/kern.post.mk

@@ -70,8 +70,9 @@ PORTSMODULESENV=\
 	-u CXX \
 	-u CPP \
 	-u MAKESYSPATH \
+	-u MK_AUTO_OBJ \
 	-u MAKEOBJDIR \
-	MAKEFLAGS="${MAKEFLAGS:M*:tW:S/^-m /-m_/g:S/ -m / -m_/g:tw:N-m_*}" \
+	MAKEFLAGS="${MAKEFLAGS:M*:tW:S/^-m /-m_/g:S/ -m / -m_/g:tw:N-m_*:NMK_AUTO_OBJ=*}" \
 	SYSDIR=${SYSDIR} \
 	PATH=${PATH}:${LOCALBASE}/bin:${LOCALBASE}/sbin \
 	SRC_BASE=${SRC_BASE} \