CONFIG_ARM_ATAG_DTB_COMPAT_CMDLINE_EXTEND is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Extend with bootloader kernel arguments

configname: CONFIG_ARM_ATAG_DTB_COMPAT_CMDLINE_EXTEND

Kernel command line type
└─>Extend with bootloader kernel arguments
In linux kernel since version 3.1 (release Date: 2011-10-24)  
The command-line arguments provided by the boot loader will be
appended to the the device tree bootargs property.