custom ubuntu core config:

gistfile1.txt

sudo /snap/bin/ubuntu-image \
     --extra-snaps=/home/cro/home_root/mnt/2/snapcraft_kernel_build_test/linuxium-kernel_4.4.0-45.66_amd64.snap \
     -c beta -o ${DIR2_}/ics-ubuntu-image.img \
     ${DIR4_}/ics.model

LOOP_DEVICE=$(sudo losetup --partscan --find --show  \
     ${DIR2_}/ics-ubuntu-image.img)
     
sudo mount ${LOOP_DEVICE}p3 ${DIR4_}/custom_image

sudo mkdir -vp ${DIR4_}/custom_image/system-data/etc/systemd/system/sysinit.target.wants
sudo ln -s /lib/systemd/system/debug-shell.service \
     ${DIR4_}/custom_image/system-data/etc/systemd/system/sysinit.target.wants/debug-shell.service
sudo mkdir -p ${DIR4_}/custom_image/system-data/var/lib/console-conf/
sudo touch ${DIR4_}/custom_image/system-data/var/lib/console-conf/complete

cat <<EOF | sudo tee ${DIR4_}/custom_image/system-data/etc/network/interfaces.d/wlan0
allow-hotplug wlan0
iface wlan0 inet dhcp
  wpa_ssid "my_custom_ssid"
  wpa_psk "my_custom_pass"
EOF

cat <<EOF | sudo tee ${DIR4_}/custom_image/system-data/etc/systemd/system/mycustom-first-boot.service
[Unit]
Description=mycustom-first-boot
ConditionPathExists=!/var/lib/console-conf/mycustom-first-boot-complete
After=network.target

[Service]
Type=forking
ExecStart=/var/lib/console-conf/mycustom-first-boot.sh
TimeoutSec=0
RemainAfterExit=yes
EOF
sudo ln -s /etc/systemd/system/mycustom-first-boot.service \
     ${DIR4_}/custom_image/system-data/etc/systemd/system/sysinit.target.wants/mycustom-first-boot.service
cat <<EOF | sudo tee ${DIR4_}/custom_image/system-data/var/lib/console-conf/mycustom-first-boot.sh
#!/bin/bash

exec >> /var/lib/console-conf/mycustom-first-boot.log
exec 2>&1
echo
date
echo

sudo adduser --extrausers --quiet --disabled-password --gecos '' mycustom

echo \"mycustom:mycustom_pass\" | chpasswd

echo 'mycustom ALL=(ALL) NOPASSWD:ALL' | sudo tee /etc/sudoers.d/mycustom

sudo mkdir -p /home/mycustom/.ssh
sudo chdown mycustom:mycustom /home/mycustom/.ssh
sudo chmod 700 /home/mycustom/.ssh
echo 'ssh-rsa mycustommycustommycustommycustommycustommycustommycustom' | sudo tee /home/mycustom/authorized_keys
sudo chown mycustom:mycustom /home/mycustom/authorized_keys
sudo 400 mycustom:mycustom /home/mycustom/authorized_keys


touch /var/lib/console-conf/mycustom-first-boot-complete

EOF
sudo chmod +x ${DIR4_}/custom_image/system-data/var/lib/console-conf/mycustom-first-boot.sh

sudo mkdir -vp ${DIR4_}/custom_image/system-data/etc/systemd/system/multi-user.target.wants
sudo ln -s /lib/systemd/system/systemd-resolved.service \
     ${DIR4_}/custom_image/system-data/etc/systemd/system/multi-user.target.wants/systemd-resolved.service