CONFIG_SOC_IMX21 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_SOC_IMX21 (not configurable)

configname: CONFIG_SOC_IMX21

Linux Kernel Configuration
└─>SOC_IMX21
In linux kernel since version 2.6.36 (release Date: 2010-10-20)  

is depended by
CONFIG_MACH_MX21ADS