CONFIG_SIMDISK0_FILENAME is not available for thedefaultarchitecture x86.
Result is shown for architecture xtensa

Host filename for the first simulated device

configname: CONFIG_SIMDISK0_FILENAME

Linux Kernel Configuration
└─>Platform options
└─>Host filename for the first simulated device
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Attach a first simdisk to a host file. Conventionally, this file
contains a root file system.