CONFIG_MACH_MX31MOBOARD is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Support mx31moboard platforms (EPFL Mobots group)

modulename: mach-mx31moboard.ko

configname: CONFIG_MACH_MX31MOBOARD

Linux Kernel Configuration
└─>Support mx31moboard platforms (EPFL Mobots group)
In linux kernel since version 2.6.29 (release Date: 2009-03-23)  
Include support for mx31moboard platform. This includes specific
configurations for the board and its peripherals.

source code:
/arch/arm/mach-mx3/

depends
CONFIG_ARCH_MX3