CONFIG_MACH_MX31ADS is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Support MX31ADS platforms

modulename: mach-mx31ads.ko

configname: CONFIG_MACH_MX31ADS

Linux Kernel Configuration
└─>Freescale i.MX family
└─>Support MX31ADS platforms
In linux kernel since version 2.6.23 (release Date: 2007-10-09)  
Include support for MX31ADS platform. This includes specific
configurations for the board and its peripherals.

source code:
arch/arm/mach-imx/mach-mx31ads.c