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

Broadcom Set-Top-Box SoCs

configname: CONFIG_ARCH_BRCMSTB

Linux Kernel Configuration
└─>Platform selection
└─>Broadcom Set-Top-Box SoCs
In linux kernel since version 3.10 (release Date: 2013-06-30)  
This enables support for Broadcom's ARMv8 Set Top Box SoCs