CONFIG_MACH_MX51_EFIKASB is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Support MX51 Genesi Efika Smartbook

modulename: board-mx51_efikasb.ko

configname: CONFIG_MACH_MX51_EFIKASB

Linux Kernel Configuration
└─>Support MX51 Genesi Efika Smartbook
In linux kernel since version 2.6.39 (release Date: 2011-05-18)  
Include support for Genesi Efika Smartbook. This includes specific
configurations for the board and its peripherals.

source code:
arch/arm/mach-mx5/board-mx51_efikasb.c