CONFIG_XTENSA_VARIANT_CUSTOM is not available for thedefaultarchitecture x86.
Result is shown for architecture xtensa

Custom Xtensa processor configuration


Xtensa Processor Configuration
└─>Custom Xtensa processor configuration
In linux kernel since version 3.17 (release Date: 2014-10-05)  
Select this variant to use a custom Xtensa processor configuration.
You will be prompted for a processor variant CORENAME.