Comment 10 for bug 736743

I think you can probably make it unattended for the time being by
editing /etc/grub.d/00_header, commenting out the following lines with a
leading # sign on each line, and then running update-grub:

  cat << EOF
  if [ -s \$prefix/grubenv ]; then
    set have_grubenv=true
    load_env
  fi
  EOF

This has the effect of disabling features like savedefault and the
system that forces the menu to be displayed if the previous boot failed;
but you aren't getting those features on btrfs right now anyway.