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

Use StiH415/416 ASC for low-level debug

configname: CONFIG_DEBUG_STI_UART

Kernel low-level debugging port
└─>Use StiH415/416 ASC for low-level debug
In linux kernel since version 3.11 (release Date: 2013-09-02)  
Say Y here if you want kernel low-level debugging support
on StiH415/416 based platforms like B2000, B2020.
It support UART2 and SBC_UART1.

If unsure, say N.