CONFIG_MACH_AT91SAM9G20EK is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Atmel AT91SAM9G20-EK Evaluation Kit

modulename: board-sam9g20ek.ko

configname: CONFIG_MACH_AT91SAM9G20EK

Linux Kernel Configuration
└─>Atmel AT91 System-on-Chip
└─>Atmel AT91SAM9G20-EK Evaluation Kit
In linux kernel since version 2.6.27 (release Date: 2008-10-09)  
Select this if you are using Atmel's AT91SAM9G20-EK Evaluation Kit
that embeds only one SD/MMC slot.

source code:
arch/arm/mach-at91/board-sam9g20ek.c