installkernel ignoring WITHOUT_DEBUG_FILES?

I'm trying to replace the kernel on memstick image with the help of 
mdconfig, and installkernel runs out of space despite 
WITHOUT_DEBUG_FILES= added to /etc/src.conf.

/usr/src$ make -V MK_DEBUG_FILES
no
/usr/src$ sudo rm -rf /mnt/usr/lib/debug
/usr/src$ sudo make DESTDIR=/mnt installkernel
--------------------------------------------------------------
 >>> Install check kernel
--------------------------------------------------------------
--------------------------------------------------------------
 >>> Installing kernel GENERIC on Wed Apr 29 08:43:19 MSK 2020
--------------------------------------------------------------
cd /usr/obj/usr/src/amd64.amd64/sys/GENERIC;  MACHINE_ARCH=amd64 
MACHINE=amd64  CPUTYPE= CC="cc -target x86_64-unknown-freebsd13.0 
--sysroot=/usr/obj/usr/src/amd64.amd64/tmp 
-B/usr/obj/usr/src/amd64.amd64/tmp/usr/bin" CXX="c++  -target 
x86_64-unknown-freebsd13.0 --sysroot=/usr/obj/usr/src/amd64.amd64/tmp 
-B/usr/obj/usr/src/amd64.amd64/tmp/usr/bin"  CPP="cpp -target 
x86_64-unknown-freebsd13.0 --sysroot=/usr/obj/usr/src/amd64.amd64/tmp 
-B/usr/obj/usr/src/amd64.amd64/tmp/usr/bin"  AS="as" AR="ar" LD="ld" 
LLVM_LINK=""  NM=nm OBJCOPY="objcopy"  RANLIB=ranlib STRINGS= 
SIZE="size" 
PATH=/usr/obj/usr/src/amd64.amd64/tmp/usr/sbin:/usr/obj/usr/src/amd64.amd64/tmp/usr/bin:/usr/obj/usr/src/amd64.amd64/tmp/legacy/usr/sbin:/usr/obj/usr/src/amd64.amd64/tmp/legacy/usr/bin:/usr/obj/usr/src/amd64.amd64/tmp/legacy/bin:/usr/obj/usr/src/amd64.amd64/tmp/legacy/usr/libexec::/sbin:/bin:/usr/sbin:/usr/bin 
make  KERNEL=kernel install thiskernel=`sysctl -n kern.bootfile` ;  if [ 
! "`dirname "$thiskernel"`" -ef /mnt/boot/kernel ] ; then  chflags
-R noschg /mnt/boot/kernel ;  rm -rf /mnt/boot/kernel ;  rm -rf 
/mnt/usr/lib/debug/boot/kernel ;  else  if [ -d /mnt/boot/kernel.old ] ; 
then  chflags -R noschg /mnt/boot/kernel.old ;  rm -rf 
/mnt/boot/kernel.old ;  fi ;  mv /mnt/boot/kernel /mnt/boot/kernel.old ; 
  if [ -n "/usr/lib/debug" -a  -d /mnt/usr/lib/debug/boot/kernel ]; then 
rm -rf /mnt/usr/lib/debug/boot/kernel.old ;  mv 
/mnt/usr/lib/debug/boot/kernel /mnt/usr/lib/debug/boot/kernel.old ;  fi 
;  sysctl kern.bootfile=/mnt/boot/kernel.old/"`basename "$thiskernel"`" 
;  fi
mkdir -p /mnt/boot/kernel
install -p -m 555 -o root -g wheel kernel /mnt/boot/kernel/
mkdir -p /mnt/usr/lib/debug/boot/kernel
install -p -m 555 -o root -g wheel kernel.debug 
/mnt/usr/lib/debug/boot/kernel/
cd /usr/src/sys/modules; 
MAKEOBJDIRPREFIX=/usr/obj/usr/src/amd64.amd64/sys/GENERIC/modules 
KMODDIR=/boot/kernel MACHINE_CPUARCH=amd64 MACHINE=amd64 
MACHINE_ARCH=amd64 MODULES_EXTRA="" WITHOUT_MODULES="" ARCH_FLAGS="" 
DEBUG_FLAGS="-g" __MPATH="" DESTDIR="/mnt" 
KERNBUILDDIR="/usr/obj/usr/src/amd64.amd64/sys/GENERIC" 
SYSDIR="/usr/src/sys" MODULE_TIED=yes WITH_CTF="1" KCSAN_ENABLED="yes" 
make  install
===> aac (install)
install -T release -o root -g wheel -m 555   aac.ko /mnt/boot/kernel/
install -T debug -o root -g wheel -m 555   aac.ko.debug 
/mnt/usr/lib/debug/boot/kernel/
...
===> dummynet (install)
install -T release -o root -g wheel -m 555   dummynet.ko /mnt/boot/kernel/

/mnt: write failed, filesystem is full
install: /mnt/boot/kernel/dummynet.ko: No space left on device