CONFIG_DEBUG_IMX6SX_UART is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

i.MX6SX Debug UART

configname: CONFIG_DEBUG_IMX6SX_UART

Kernel low-level debugging port
└─>i.MX6SX Debug UART
In linux kernel since version 3.16 (release Date: 2014-08-03)  
Say Y here if you want kernel low-level debugging support
on i.MX6SX.