CONFIG_DEBUG_BRCMSTB_UART is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Use BRCMSTB UART for low-level debug

configname: CONFIG_DEBUG_BRCMSTB_UART

Kernel low-level debugging port
└─>Use BRCMSTB UART for low-level debug
In linux kernel since version 3.19 (release Date: 2015-02-08)  
Say Y here if you want the debug print routines to direct
their output to the first serial port on these devices.

If you have a Broadcom STB chip and would like early print
messages to appear over the UART, select this option.