CONFIG_MACH_SMDK6400 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

SMDK6400

modulename: mach-smdk6400.ko

configname: CONFIG_MACH_SMDK6400

Linux Kernel Configuration
└─>Samsung S3C64XX (deprecated, see help)
└─>SMDK6400
In linux kernel since version 2.6.31 (release Date: 2009-09-09)  
Machine support for the Samsung SMDK6400