CONFIG_DEBUG_IMX21_IMX27_UART is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_DEBUG_IMX21_IMX27_UART.
Result is shown for kernelversion 6.7-rc3

i.MX21 and i.MX27 Debug UART

configname: CONFIG_DEBUG_IMX21_IMX27_UART

Kernel low-level debugging port
└─>i.MX21 and i.MX27 Debug UART
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Say Y here if you want kernel low-level debugging support
on i.MX21 or i.MX27.