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.
I think you can probably make it unattended for the time being by d/00_header, commenting out the following lines with a
editing /etc/grub.
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.