CONFIG_MACH_MX21ADS is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

MX21ADS platform

modulename: mx21ads.ko

and/or

mach-mx21ads.ko

configname: CONFIG_MACH_MX21ADS

Linux Kernel Configuration
└─>MX21ADS platform
In linux kernel since version 2.6.31 (release Date: 2009-09-09)  
Include support for MX21ADS platform. This includes specific
configurations for the board and its peripherals.

source code:
arch/arm/mach-mx2/mx21ads.c