Document how to boot backup vmlinuz.old

[mwip]

When trying to revert back to an old image (see tobetter/linux#30 for reason :) ) I found no documentation on how to boot from a backup image.

This should absolutely be added to this documentation as the Odroid images do not use the standard grub2 setup.

[mwip]

So I managed to find a way that reboots my machine. Yet, as I am familiar with uBoot (if that is even installed used on the HC4 Debian image) I kindly ask for confirmation:

(as root)

# USE AT OWN YOUR RISK!
# cp /boot/boot.src /boot/boot.src.new-kernel-corrupted-backup
# cp /boot/boot.src.bak /boot/boot.src

@tobetter I am totally willing to provide a PR in case this would be helpful.