CONFIG_UML is not available for thedefaultarchitecture x86.
Result is shown for architecture um

CONFIG_UML (not configurable)

configname: CONFIG_UML

Linux Kernel Configuration
└─>UML
In linux kernel since version 2.6.12  
selects
CONFIG_HAVE_GENERIC_HARDIRQS
CONFIG_HAVE_UID16
CONFIG_GENERIC_IRQ_SHOW
CONFIG_GENERIC_CPU_DEVICES
CONFIG_GENERIC_IO
CONFIG_GENERIC_CLOCKEVENTS
CONFIG_TTY