CONFIG_S3C2440_XTAL_12000000 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_S3C2440_XTAL_12000000 (not configurable)

configname: CONFIG_S3C2440_XTAL_12000000

Linux Kernel Configuration
└─>S3C2440_XTAL_12000000
In linux kernel since version 2.6.32 (release Date: 2009-12-02)  
Indicate that the build needs to support 12MHz system
crystal.

depends
 or CONFIG_ARCH_S3C24XX |
| CONFIG_CPU_S3C2440 or CONFIG_CPU_S3C2440 or CONFIG_CPU_S3C2442