CONFIG_MACH_MX21ADS is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_MACH_MX21ADS.
Result is shown for kernelversion 6.8.2

MX21ADS platform

modulename: mach-mx21ads.ko

configname: CONFIG_MACH_MX21ADS

Linux Kernel Configuration
└─>Freescale i.MX family
└─>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: