CONFIG_ATH79_MACH_AP121 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

Atheros AP121 reference board

modulename: mach-ap121.ko

configname: CONFIG_ATH79_MACH_AP121

Linux Kernel Configuration
└─>Atheros AR71XX/AR724X/AR913X machine selection
└─>Atheros AP121 reference board
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Say 'Y' here if you want your kernel to support the
Atheros AP121 reference board.

source code:
arch/mips/ath79/mach-ap121.c

depends
CONFIG_ATH79