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

CONFIG_DEBUG_STI_UART (not configurable)

configname: CONFIG_DEBUG_STI_UART

Linux Kernel Configuration
└─>DEBUG_STI_UART
In linux kernel since version 3.11 (release Date: 2013-09-02)  
depends
CONFIG_ARCH_STI