CONFIG_ARCH_THUNDER2 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm64

Cavium ThunderX2 Server Processors

configname: CONFIG_ARCH_THUNDER2

Linux Kernel Configuration
└─>Platform selection
└─>Cavium ThunderX2 Server Processors
In linux kernel since version 4.2 (release Date: 2015-08-30)  
This enables support for Cavium's ThunderX2 CN99XX family of
server processors.