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

Broadcom SoC

modulename: board_bcm.ko

configname: CONFIG_ARCH_BCM

Linux Kernel Configuration
└─>Broadcom SoC
In linux kernel since version 3.1 (release Date: 2011-10-24)  
This enables support for system based on Broadcom SoCs.
It currently supports the 'BCM281XX' family, which includes
BCM11130, BCM11140, BCM11351, BCM28145 and
BCM28155 variants.

source code:
arch/arm/mach-bcm/board_bcm.c


is selected by
CONFIG_ARCH_BCM2835