Or alternatively, curtin could write /etc/default/grub.d/50-curtin.cfg as: GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT <curtin specific options if any>" So the variable in /etc/default/grub will be also respected by update-grub.
Or alternatively, curtin could write /etc/default/ grub.d/ 50-curtin. cfg as: LINUX_DEFAULT= "$GRUB_ CMDLINE_ LINUX_DEFAULT <curtin specific options if any>"
GRUB_CMDLINE_
So the variable in /etc/default/grub will be also respected by update-grub.