CONFIG_ATH79_MACH_AP81 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

Atheros AP81 reference board

modulename: mach-ap81.ko

configname: CONFIG_ATH79_MACH_AP81

Linux Kernel Configuration
└─>Atheros AR71XX/AR724X/AR913X machine selection
└─>Atheros AP81 reference board
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  
Say 'Y' here if you want your kernel to support the
Atheros AP81 reference board.

source code:
arch/mips/ath79/mach-ap81.c

depends
CONFIG_ATH79