buildkernel with PORTS_MODULES fails: Variable OBJTOP is recursive

Hello!

For some time (about a week) building and installing kernel fails with
the error "Variable OBJTOP is recursive." when going to build/install
module from ports.

Last successful build was at r328426. Next build at r328527 failed and
still broken at r328649.

Without PORTS_MODULES building and installing kernel succeeds. Another
workaround: ignore error and build/install module directly from ports.

# cat /etc/make.conf
MALLOC_PRODUCTION=yes
KERNCONF=GENERIC-NODEBUG GENERIC
#KERNCONF= GENERIC-NODEBUG
CPUTYPE?=native
#PORTS_MODULES = graphics/drm-next-kmod emulators/virtualbox-ose-kmod
PORTS_MODULES = graphics/drm-next-kmod 

DOC_LANG = en_US.ISO8859-1 ru_RU.KOI8-R 

WITH_DEBUG_PORTS = mail/neomutt

WITH_CCACHE_BUILD=yes
CCACHE_DIR=/home/ccache

#DEVELOPER=yes

...
Building /home/obj/usr/src/amd64.amd64/sys/GENERIC-NODEBUG/kernel.full
--- kernel.full ---
linking kernel.full
ctfmerge -L VERSION -g -o kernel.full ...
      text      data       bss        dec         hex   filename
  22584632   1376209   4748888   28709729   0x1b61361   kernel.full
Building /home/obj/usr/src/amd64.amd64/sys/GENERIC-NODEBUG/kernel.debug
Building /home/obj/usr/src/amd64.amd64/sys/GENERIC-NODEBUG/kernel
--- all ---
===> Ports module graphics/drm-next-kmod (all)
cd ${PORTSDIR:-/usr/ports}/graphics/drm-next-kmod; env  -u CC  -u CXX
-u CPP  -u MAKESYSPATH  MAKEFLAGS="-j 4 -J 15,16 -j 4 -J 15,16 -D
NO_MODULES_OBJ .MAKE.LEVEL.ENV=MAKELEVEL KERNEL=kernel TARGET=amd64
TARGET_ARCH=amd64"  SYSDIR=/usr/src/sys
PATH=<long string omitted>
SRC_BASE=/usr/src  OSVERSION=1200056
WRKDIRPREFIX=/home/obj/usr/src/amd64.amd64/sys/GENERIC-NODEBUG make -B
clean build
===>  Cleaning for drm-next-kmod-g20180117
===>  License BSD2CLAUSE MIT GPLv2 accepted by the user
===>   drm-next-kmod-g20180117 depends on file: /usr/local/sbin/pkg - found
===> Fetching all distfiles required by drm-next-kmod-g20180117 for building
===>  Extracting for drm-next-kmod-g20180117
=> SHA256 Checksum OK for FreeBSDDesktop-kms-drm-g20180117-622fdd1_GH0.tar.gz.
===>  Patching for drm-next-kmod-g20180117
===>   drm-next-kmod-g20180117 depends on file: /usr/local/bin/ccache - found
===>  Configuring for drm-next-kmod-g20180117
===>  Building for drm-next-kmod-g20180117
===> drm (all)
Variable OBJTOP is recursive.

make[6]: stopped in <long path omitted>
*** Error code 2

Stop.
make[5]: stopped in <long path omitted>
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure to
the maintainer.
*** Error code 1

Stop.
make[4]: stopped in /home/ports/graphics/drm-next-kmod
*** Error code 1

Stop.
make[3]: stopped in /home/ports/graphics/drm-next-kmod
*** [all] Error code 1
...

-- 
Regards,                 | "In theory there is no difference between theory
  Vladimir Zakharov      | and practice. In practice there is."- Yogi Berra