CONFIG_DEFAULT_MEM_START is not available for thedefaultarchitecture x86.
Result is shown for architecture xtensa

PAGE_OFFSET/PHYS_OFFSET

configname: CONFIG_DEFAULT_MEM_START

Linux Kernel Configuration
└─>Platform options
└─>PAGE_OFFSET/PHYS_OFFSET
In linux kernel since version 3.19 (release Date: 2015-02-08)  
This is the base address used for both PAGE_OFFSET and PHYS_OFFSET
in noMMU configurations.

If unsure, leave the default value here.