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

i.MX21 support

modulename: clock-imx21.ko

configname: CONFIG_MACH_MX21

CPUs:
└─>i.MX21 support
In linux kernel since version 2.6.30 (release Date: 2009-06-09)  
This enables support for Freescale's MX2 based i.MX21 processor.

source code:
/arch/arm/mach-imx/

is selected by
CONFIG_MACH_MX21ADS