CONFIG_MACH_SMDK4212 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

SMDK4212

modulename: mach-smdk4x12.ko

configname: CONFIG_MACH_SMDK4212

Linux Kernel Configuration
└─>SAMSUNG EXYNOS SoCs Support
└─>SMDK4212
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Machine support for Samsung SMDK4212

source code:
arch/arm/mach-exynos/mach-smdk4x12.c