What's powerpc-ieee1275's equivalent for chainloader?

[Vax456]

I have a netboot directory for powerpc-ieee1275, which is being used for
old PowerPC Macintosh's.  Netbooting into Grub2 works, but I need a way
to boot from the first disk.

On i386 machines, this can be accomplished using:

insmod nativedisk
insmod regexp
for disk in (*); do
        set root=${disk}
        chainloader +1
        if [ $? = 0 ]; then
                boot
        fi
done

However, PowerPC doesn't have chainloader, so the above won't work.  So
what other method can be used for booting on the first disk on PowerPC?