CONFIG_DEBUG_LL_UART_8250 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Kernel low-level debugging via 8250 UART

configname: CONFIG_DEBUG_LL_UART_8250

Kernel low-level debugging port
└─>Kernel low-level debugging via 8250 UART
In linux kernel since version 3.12 (release Date: 2013-11-03)  
Say Y here if you wish the debug print routes to direct
their output to an 8250 UART. You can use this option
to provide the parameters for the 8250 UART rather than
selecting one of the platform specific options above if
you know the parameters for the port.

This option is preferred over the platform specific
options; the platform specific options are deprecated
and will be soon removed.