CONFIG_ARCH_MX50 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_ARCH_MX50 (not configurable)

configname: CONFIG_ARCH_MX50

Linux Kernel Configuration
└─>ARCH_MX50
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  

is depended by
CONFIG_SOC_IMX50