CONFIG_ATH79_MACH_AP81 is not available for the default architecture 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

Say 'Y' here if you want your kernel to support the
Atheros AP81 reference board.

source code: