In /etc/kernel-img.conf:
postinst_hook = /sbin/update-grub
postrm_hook = /sbin/update-grub
do_bootloader = No
do_initrd = Yes
do_bootloader needs to be No counterintuitively, because “bootloader” actually means LILO, and we aren't using LILO. do_initrd = Yes suppresses the warning about configuring your grub menu.lst for use with an initrd kernel.