There is nothing different introduced by Debian: install extlinux as usual with:
extlinux --install /boot/extlinux
make sure kernel image is present with matching initrd;
create a /boot/extlinux/extlinux.conf file like:
default linux
timeout 1
label linux
linux ../vmlinuz-4.4.0
append initrd=../initrd.img-4.4.0 root=/dev/vda1 console=tty0 console=ttyS0,115200 ro quiet
label linux-stock
kernel ../vmlinuz-3.16.0-4-amd64
append initrd=../initrd.img-3.16.0-4-amd64 root=/dev/vda1 console=tty0 console=ttyS0,115200 ro quiet
note the path are right in a box in which /boot is not a partition on its own (it is part of / partition).
If the kernel image is too large, use the keyword linux (not kernel).