CONFIG_PPC_MPC5121 is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

CONFIG_PPC_MPC5121 (not configurable)

configname: CONFIG_PPC_MPC5121

Linux Kernel Configuration
└─>PPC_MPC5121
In linux kernel since version 2.6.25 (release Date: 2008-04-16)