CONFIG_XTENSA_VARIANT_FSF is not available for the selected architecture x86.
Result is shown for architecture xtensa

fsf - default (not generic) configuration

configname: CONFIG_XTENSA_VARIANT_FSF

Xtensa Processor Configuration
└─>fsf - default (not generic) configuration
In linux kernel since version 2.6.20 (release Date: 2007-02-04)
selects
CONFIG_MMU