The attached pdf descibes the method in very brief form of how i used the MPC5121 LTIB as the basis for the MPC5125 LTIB. By adding the MPC5155 BSP to the MPC5121 LTIB and re-configuring the LTIB, you can end up with a compilable MPC5125 kernel.
I have used this to build the kernel on a Suse 11.3 virtual machine. It has been run sucessfully and I hope that this rough guide helps.
I would be interested in any comments or questions as these forums help me as much as anybody else.