CONFIG_DEBUG_IMX6SL_UART is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

i.MX6SL Debug UART

configname: CONFIG_DEBUG_IMX6SL_UART

Kernel low-level debugging port
└─>i.MX6SL Debug UART
In linux kernel since version 3.11 (release Date: 2013-09-02)  
Say Y here if you want kernel low-level debugging support
on i.MX6SL.