CONFIG_ATH79_MACH_AP121 is not available for the default architecture 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

Say 'Y' here if you want your kernel to support the
Atheros AP121 reference board.

source code: