CONFIG_MPC8641 is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

CONFIG_MPC8641 (not configurable)

configname: CONFIG_MPC8641

Linux Kernel Configuration
└─>Platform Support
└─>MPC8641
In linux kernel since version 2.6.18 (release Date: 2006-09-19)  

depends
CONFIG_PPC_86xx