CONFIG_KERNEL_START_BOOL is not available for the selected architecture x86.
Result is shown for architecture microblaze

Set custom kernel base address

configname: CONFIG_KERNEL_START_BOOL

Linux Kernel Configuration
└─>Kernel features
└─>Set custom kernel base address
In linux kernel since version 2.6.15 (release Date: 2006-01-02)
This option allows you to set the kernel virtual address at which
the kernel will map low memory (the kernel image will be linked at
this address). This can be useful in optimizing the virtual memory
layout of the system.

Say N here unless you know what you are doing.