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

CONFIG_PPC_RTAS (not configurable)

modulename: rtas.ko

configname: CONFIG_PPC_RTAS

Linux Kernel Configuration
└─>Platform support
└─>PPC_RTAS
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
source code:
arch/powerpc/kernel/