CONFIG_MPC836x_RDK is not available for the selected architecture x86.
Result is shown for architecture powerpc

Freescale/Logic MPC836x RDK

modulename: mpc836x_rdk.ko

configname: CONFIG_MPC836x_RDK

Linux Kernel Configuration
└─>83xx-based boards
└─>Freescale/Logic MPC836x RDK
In linux kernel since version 2.6.27 (release Date: 2008-10-09)
This option enables support for the MPC836x RDK Processor Board,
also known as ZOOM PowerQUICC Kit.
source code:
arch/powerpc/platforms/83xx/mpc836x_rdk.c

depends
CONFIG_PPC_83xx