CONFIG_DEBUG_IMX31_IMX35_UART is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_DEBUG_IMX31_IMX35_UART.
Result is shown for kernelversion 5.3.7

i.MX31 and i.MX35 Debug UART


Kernel low-level debugging port
└─>i.MX31 and i.MX35 Debug UART
In linux kernel since version 3.3 (release Date: 2012-03-18)  
Say Y here if you want kernel low-level debugging support
on i.MX31 or i.MX35.