I decided to try out kernel-bfs.
After trying to install, it gave me that there are conflicts with some package from the kernel-power and decided to remove kernel-power.

So I restored the Nokia kernel (from the menu icon), removed the bootimg and did a reboot.
After that, I could still select the power48 option (while I actually removed the bootimg!)

But, that didn't do anything, so I picked stock kernel.

After that, I installed kernel-bfs, and the bootimg of bfs.
But it didn't show up in my bootmenu.

I put the .item file manually into my phone, selected the file, but it doesn't boot anymore. Neither into the stock kernel.

After flashing the kernel-only, I could boot up back into stock.
So, I removed everything from kernel-bfs (also manually the .item file) and decided to put kernel-power (48) back with the bootimg.

Guess what? There was no bootmenu entry for kernel power!?

So again, I added manually the .list file... and the result: Same problem as I had with kernel-bfs: I coudlnt boot anymore into any of my kernels.

So I flashed the kernel again, now I can boot into the stock...

Does anyone know what I should do to fix this problem?

Edit:
Hmm, I tried one more time and it did work after manually adding the .list file and the zImage file.

Altough I find it odd that it doesn't install those files anymore (I had to place them manually in their correct places)