I havent fiddled a lot with bootloaders, but you would have to install a bootloader and have an entry that points to the vmlinuz on the sd card.

A bootloader you can at least find in quole's repository, and most if not all load straight into maemo if your keyboard is not out, and if you have your keyboard out you can select between the OS's you have made bootloader entries for.

The reason I have not fiddled with bootloaders is that on computers they usually just configure them self, but this will not be the case in this scenario.

EDIT:
To answer your question more correctly, but im not entirely sure, it would be something like root = "/sd2/boot/vmlinuz" or something like that...