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

Use BRCMSTB UART for low-level debug


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. The
UART physical and virtual address is automatically provided
based on the chip identification register value.

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