CONFIG_MACH_MX51_EFIKAMX is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Support MX51 Genesi Efika MX nettop

modulename: board-mx51_efikamx.ko

configname: CONFIG_MACH_MX51_EFIKAMX

Linux Kernel Configuration
└─>Support MX51 Genesi Efika MX nettop
In linux kernel since version 2.6.37 (release Date: 2011-01-04)  
Include support for Genesi Efika MX nettop. This includes specific
configurations for the board and its peripherals.

source code:
arch/arm/mach-mx5/board-mx51_efikamx.c