CONFIG_MPC5121_ADS is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

Freescale MPC5121E ADS

modulename: mpc5121_ads.ko

configname: CONFIG_MPC5121_ADS

Linux Kernel Configuration
└─>Freescale MPC5121E ADS
In linux kernel since version 2.6.25 (release Date: 2008-04-16)  
This option enables support for the MPC5121E ADS board.

source code:
/arch/powerpc/platforms/512x/
selects
CONFIG_DEFAULT_UIMAGE
CONFIG_MPC5121_ADS_CPLD