CONFIG_MACH_MX51_3DS is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Support MX51PDK (3DS)

modulename: mach-mx51_3ds.ko

configname: CONFIG_MACH_MX51_3DS

Linux Kernel Configuration
└─>Freescale i.MX support
└─>Support MX51PDK (3DS)
In linux kernel since version 2.6.36 (release Date: 2010-10-20)  
Include support for MX51PDK (3DS) platform. This includes specific
configurations for the board and its peripherals.

source code:
arch/arm/mach-imx/mach-mx51_3ds.c