CONFIG_CACHE_B15_RAC is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Enable the Broadcom Brahma-B15 read-ahead cache controller

modulename: cache-b15-rac.ko

configname: CONFIG_CACHE_B15_RAC

Linux Kernel Configuration
└─>Enable the Broadcom Brahma-B15 read-ahead cache controller
In linux kernel since version 3.10 (release Date: 2013-06-30)  
This option enables the Broadcom Brahma-B15 read-ahead cache
controller. If disabled, the read-ahead cache remains off.

source code: