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

PowerPC Reference Platform (PReP) based machines

configname: CONFIG_PPC_PREP

Linux Kernel Configuration
└─>PowerPC Reference Platform (PReP) based machines
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  

depends
CONFIG_6xx
CONFIG_BROKEN