Yeah it is simple your kernel image is zImage-2.6.28.10-power47 and your modules directory is 2.6.28.10-power47-g87cd160-dirty and in .item file you have line ITEM_KERNEL="2.6.28.10-power47". I wrote in bold parts which should be the same for correct multiboot boot So you have two choices:
1. simpler and more proper (if you would like to have later "clean" power kernel 47 installed for example): rename /boot/zImage-2.6.28.10-power47 to /boot/zImage-2.6.28.10-power47-g87cd160-dirty and change line in .item file to ITEM_KERNEL="2.6.28.10-power47-g87cd160-dirty"
2. dirtier (you will not be able to use kernel power 47 default files at the same time): rename /lib/modules/2.6.28.10-power47-g87cd160-dirty directory to /lib/modules/2.6.28.10-power47 and don't modify rest.