CONFIG_WANT_DEVICE_TREE is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

CONFIG_WANT_DEVICE_TREE (not configurable)

configname: CONFIG_WANT_DEVICE_TREE

Linux/PowerPC Kernel Configuration
└─>Kernel options
└─>WANT_DEVICE_TREE
In linux kernel since version 2.6.23 (release Date: 2007-10-09)