CONFIG_MACH_NSLU2 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Linksys NSLU2

modulename: nslu2-setup.ko

configname: CONFIG_MACH_NSLU2

Linux Kernel Configuration
└─>Intel IXP4xx Implementation Options
└─>Linksys NSLU2
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
Say 'Y' here if you want your kernel to support Linksys's
NSLU2 NAS device. For more information on this platform,
see http://www.nslu2-linux.org

source code:
arch/arm/mach-ixp4xx/nslu2-setup.c
selects
CONFIG_PCI