Well, this was more than two and a half year ago. I don’t remember exactly what I did that made it work but here is the .bb-file that must be the right one:
DESCRIPTION = "A console-only image with more full-featured Linux system \
functionality installed."
inherit core-image
CONMANPKGS ?= "\
connman \
connman-plugin-loopback \
connman-plugin-ethernet \
connman-plugin-wifi \
connman-client"
IMAGE_FEATURES += "\
splash \
ssh-server-openssh \
"
IMAGE_INSTALL = "\
packagegroup-core-boot \
packagegroup-core-full-cmdline \
${CORE_IMAGE_EXTRA_INSTALL} \
\
${CONMANPKGS} \
nano \
openssh-sftp-server \
"