CONFIG_MACH_NSA310_DT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_MACH_NSA310_DT.
Result is shown for kernelversion 6.2.5

ZyXEL NSA-310 (Flattened Device Tree)

modulename: board-nsa310.ko

configname: CONFIG_MACH_NSA310_DT

Linux Kernel Configuration
└─>Marvell Kirkwood Implementations
└─>ZyXEL NSA-310 (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
ZyXEL NSA-310 board (Flattened Device Tree).

source code:
arch/arm/mach-kirkwood/board-nsa310.c