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

Simtec Electronics BAST (EB2410ITX)

modulename: mach-bast.ko

configname: CONFIG_ARCH_BAST

Linux Kernel Configuration
└─>SAMSUNG S3C24XX SoCs Support
└─>Simtec Electronics BAST (EB2410ITX)
In linux kernel since version 2.6.12  
Say Y here if you are using the Simtec Electronics EB2410ITX
development board (also known as BAST)

source code:
arch/arm/mach-s3c24xx/mach-bast.c