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

Firmware flash interface

modulename: rtas_flash.ko

configname: CONFIG_RTAS_FLASH

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