CONFIG_MPC85xx_RDB is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

Freescale P102x MBG/UTM/RDB and P2020 RDB

modulename: mpc85xx_rdb.ko

configname: CONFIG_MPC85xx_RDB

Linux Kernel Configuration
└─>Freescale Book-E Machine Type
└─>Freescale P102x MBG/UTM/RDB and P2020 RDB
In linux kernel since version 2.6.32 (release Date: 2009-12-02)  
This option enables support for the P1020 MBG PC, P1020 UTM PC,
P1020 RDB PC, P1020 RDB PD, P1020 RDB, P1021 RDB PC, P1024 RDB,
P1025 RDB, P2020 RDB and P2020 RDB PC boards

source code:
arch/powerpc/platforms/85xx/mpc85xx_rdb.c