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

Support for the traditional ATAGS boot data passing

modulename: atags_parse.ko

configname: CONFIG_ATAGS

Linux Kernel Configuration
└─>Boot options
└─>Support for the traditional ATAGS boot data passing
In linux kernel since version 3.1 (release Date: 2011-10-24)  
This is the traditional way of passing data to the kernel at boot
time. If you are solely relying on the flattened device tree (or
the ARM_ATAG_DTB_COMPAT option) then you may unselect this option
to remove ATAGS support from your kernel binary. If unsure,
leave this to y.

source code:
arch/arm/kernel/atags_parse.c