grub-mkconfig always generates a devicetree line when a dtb is present, causing a boot delay when grub is in lockdown

When a device tree is found in /boot, grub-mkconfig automatically appends a devicetree line to the menu entry to load the device tree alongside the initrd and kernel.

Excerpt from /etc/grub.d/10_linux from grub-2.12:

    if test -n "${dtb}" ; then
      if [ x"$quiet_boot" = x0 ] || [ x"$type" != xsimple ]; then
        message="$(gettext_printf "Loading device tree blob...")"
        sed "s/^/$submenu_indentation/" << EOF
        echo '$(echo "$message" | grub_quote)'
EOF
      fi
      sed "s/^/$submenu_indentation/" << EOF
        devicetree ${rel_dirname}/${dtb}
EOF
    fi

This is however not always desired, in particular when GRUB is in lockdown (e.g. due to Secure Boot), as the devicetree command is then forbidden. This causes a ~5 second boot delay during startup as the following message is shown:

error: devicetree: the command is not allowed when lockdown is enforced.

Press any key to continue...

GRUB continues to boot without a key being pressed, but this is an unnecessary delay when booting. I suggest adding an additional variable (e.g. GRUB_DISABLE_DEVICETREE) that can be set in /etc/default/grub to tell GRUB that it should not add the devicetree command.

I don't think this is something that can be automatically detected and accounted for during grub-mkconfig, as the command may not be being executed on the target, and it may not know that the target will be booting with a locked down GRUB.