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

Broadcom Mobile SoC

modulename: board_bcm281xx.ko

configname: CONFIG_ARCH_BCM_MOBILE

Linux Kernel Configuration
└─>Broadcom SoC Selection
└─>Broadcom Mobile SoC
In linux kernel since version 3.10 (release Date: 2013-06-30)  
This enables support for systems based on Broadcom mobile SoCs.
It currently supports the 'BCM281XX' family, which includes
BCM11130, BCM11140, BCM11351, BCM28145 and
BCM28155 variants.

source code:
/arch/arm/mach-bcm/