CONFIG_BUILTIN_DTB_SOURCE is not available for thedefaultarchitecture x86.
Result is shown for architecture sh
default or selected kernelversion does not have config value CONFIG_BUILTIN_DTB_SOURCE.
Result is shown for kernelversion 6.9.9

Source file for builtin DTB

configname: CONFIG_BUILTIN_DTB_SOURCE

Linux Kernel Configuration
└─>Boot options
└─>Source file for builtin DTB
In linux kernel since version 3.10 (release Date: 2013-06-30)  
Base name (without suffix, relative to arch/sh/boot/dts) for the
a DTS file that will be used to produce the DTB linked into the
kernel.