CONFIG_29BIT is not available for thedefaultarchitecture x86.
Result is shown for architecture sh

CONFIG_29BIT (not configurable)

configname: CONFIG_29BIT

Linux Kernel Configuration
└─>Memory management options
└─>29BIT
In linux kernel since version 2.6.25 (release Date: 2008-04-16)  

depends
CONFIG_SUPERH32