I can't boot kp51 with U-Boot. It gets to "starting kernel", then screen goes black, powers off. My U-Boot version is "U-Boot with Nokia PR1.3 kernel 2012.04-1" Maybe there is a newer version?
The stock kernel boots fine.
I had kp50 before that, in U-Boot bootimage, this used to work.
After updating extras-devel, kp51 became available in HAM, but kernel-power-bootimg is not there - I was able to install it via apt-get though.