Booting from the rescue system located on a hard disk partition.

In this example /dev/sda2 is mounted on /rescue when the system is running normally.

Here we are using the rescue system from Leap 42.1.

Populate /rescue

mount openSUSE-Leap-42.1-DVD-x86_64.iso /mnt
cd /mnt
rsync -avv --exclude='suse' . /rescue

Create a custom grub entry

Edit /etc/grub.d/40_custom so that it looks like this (of course there may be other custom menuentry sections).

exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
menuentry 'Rescue System' --class opensuse --class gnu-linux --class gnu {
    set gfxpayload=keep
    echo 'Loading kernel ...'
    set root='hd0,gpt2'
    linuxefi /boot/x86_64/loader/linux rescue=1
    echo 'Loading initial ramdisk ...'
    initrdefi /boot/x86_64/loader/initrd

(In grub2, counting disks starts from 0, while counting partitions starts from 1.)

Rebuild the grub menu

Then grub2-mkconfig -o /boot/grub2/grub.cfg