CONFIG_MACH_SMDK6450 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

SMDK6450

modulename: mach-smdk6450.ko

configname: CONFIG_MACH_SMDK6450

Linux Kernel Configuration
└─>SMDK6450
In linux kernel since version 2.6.37 (release Date: 2011-01-04)  
Machine support for the Samsung SMDK6450

source code:
arch/arm/mach-s5p64x0/mach-smdk6450.c