Remove GRUB and install extlinux:
apt purge grub-pc grub-common grub-pc-bin grub2-common && rm -rf /boot/grub/ && apt install extlinux syslinux-common
If /boot is not on separate partition:
extlinux --install /boot
Otherwise:
extlinux --install /boot/extlinux
Overwrite GRUBs master boot record:
dd if=/usr/lib/syslinux/mbr/mbr.bin of=/dev/sda bs=440 count=1
For boot menu copy:
cp /usr/lib/syslinux/modules/bios/menu.c32 / # text menu
OR
cp /usr/lib/syslinux/modules/bios/vesamenu.c32 / # graphical menu
cp /usr/lib/syslinux/modules/bios/lib* /
Create /extlinux.conf in / . Example conf:
default linuxAAA label linuxAAA kernel vmlinuz append initrd=initrd.img root=/dev/sda1 ro quiet label stari kernel kernel vmlinuz.old append initrd=initrd.img.old root=/dev/sda1 ro quiet timeout 60 ui vesamenu.c32
If stuck in boot> menu prompt use:
vmlinuz initrd=initrd.img root=/dev/sda1
to boot images linked in / directory on /dev/sda1.