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
└─>SMDK6400
In linux kernel since version 2.6.31 (release Date: 2009-09-09)
Machine support for the Samsung SMDK6400