CONFIG_BOOT_ELF64 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_BOOT_ELF64 (not configurable)

configname: CONFIG_BOOT_ELF64

Linux/MIPS Kernel Configuration
└─>BOOT_ELF64
In linux kernel since version 2.6.12  
is depended by
CONFIG_SGI_IP28