Comment 5 for bug 1652091

I have worked around this in my Xenial dropbear/initrd environment by replacing the package /usr/share/initramfs-tools/scripts/init-bottom/dropbear with:

##############################################
#!/bin/sh
#From /usr/share/initramfs-tools/scripts/init-bottom/dropbear

PREREQ=""

prereqs() {
 echo "$PREREQ"
}

case "$1" in
 prereqs)
  prereqs
  exit 0
 ;;
esac

. /scripts/functions

[ -s /run/dropbear.pid ] || exit 0

log_begin_msg "Stopping dropbear and its children"

pid=$(cat /run/dropbear.pid)
for child in `ps l | awk '$4=="'$pid'"{print $3}'`; do
  kill $child
done
kill $pid
##############################################

Hope that might help someone.