I forgot to upload the systemd service file earlier.
Install this using a systemd_%.bbappend:
FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
SRC_URI:append = " \
file://opensprinkler.service \
"
FILES:${PN}:append = " \
${systemd_unitdir}/system/opensprinkler.service \
"
do_install:append() {
# Install opensprinkler
install -m 0644 ${UNPACKDIR}/opensprinkler.service ${D}/${systemd_unitdir}/system
ln --relative --symbolic ${D}${systemd_unitdir}/system/opensprinkler.service ${D}${systemd_unitdir}/system/multi-user.target.wants/opensprinkler.service
}