CONFIG_ATH79_MACH_DB120 is not available for the default architecture x86.
Result is shown for architecture mips

Atheros DB120 reference board

modulename: mach-db120.ko
configname: CONFIG_ATH79_MACH_DB120

Linux Kernel Configuration
└─> Atheros AR71XX/AR724X/AR913X machine selection
└─> Atheros DB120 reference board

Say 'Y' here if you want your kernel to support the
Atheros DB120 reference board.

source code: