CONFIG_MACH_KM_KIRKWOOD_DT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Keymile Kirkwood Reference Design (Flattened Device Tree)

modulename: board-km_kirkwood.ko

configname: CONFIG_MACH_KM_KIRKWOOD_DT

Linux Kernel Configuration
└─>Marvell Kirkwood Implementations
└─>Keymile Kirkwood Reference Design (Flattened Device Tree)
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Say 'Y' here if you want your kernel to support the
Keymile Kirkwood Reference Desgin, using Flattened Device Tree.

source code:
arch/arm/mach-kirkwood/board-km_kirkwood.c