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

CONFIG_DEFAULT_UIMAGE (not configurable)

configname: CONFIG_DEFAULT_UIMAGE

Linux/PowerPC Kernel Configuration
└─>DEFAULT_UIMAGE
In linux kernel since version 2.6.16 (release Date: 2006-03-19)  
Used to allow a board to specify it wants a uImage built by default