CONFIG_MACH_IXDP2351 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Support Intel IXDP2351 platform

modulename: ixdp2351.ko

configname: CONFIG_MACH_IXDP2351

Linux Kernel Configuration
└─>Intel IXP23xx Implementation Options
└─>Support Intel IXDP2351 platform
In linux kernel since version 2.6.17 (release Date: 2006-06-17)  
source code:
arch/arm/mach-ixp23xx/ixdp2351.c