CONFIG_STM32F4_DEBUG_UART is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Use STM32F4 UART for low-level debug

configname: CONFIG_STM32F4_DEBUG_UART

Kernel low-level debugging port
└─>Use STM32F4 UART for low-level debug
In linux kernel since version 5 (release Date: 2019-03-03)  
Say Y here if you want kernel low-level debugging support
on STM32F4 based platforms, which default UART is wired on
USART1.

If unsure, say N.