CONFIG_SOC_IMX51 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
i.MX51 support
modulename: mach-imx51.ko
configname: CONFIG_SOC_IMX51
Linux Kernel Configuration
└─>Freescale i.MX family
└─>i.MX51 support
In linux kernel since version 2.6.38 (release Date: 2011-03-14)
This enables support for Freescale i.MX51 processor