CONFIG_NIOS2_MEM_BASE is not available for thedefaultarchitecture x86.
Result is shown for architecture nios2

Memory base address


Linux Kernel Configuration
└─>Platform options
└─>Memory base address
In linux kernel since version 3.19 (release Date: 2015-02-08)  
This is the physical address of the memory that the kernel will run
from. This address is used to link the kernel and setup initial memory
management. You should take the raw memory address without any MMU
or cache bits set.
Please not that this address is used directly so you have to manually
do address translation if it's connected to a bridge.