CONFIG_MACH_MX21 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_MACH_MX21 (not configurable)

configname: CONFIG_MACH_MX21

Linux Kernel Configuration
└─>MACH_MX21
In linux kernel since version 2.6.30 (release Date: 2009-06-09)  
is depended by
CONFIG_SOC_IMX21