[Buildroot] how to make the Buildroot embedded linux file system persistence?

[Amir Amir]

I have built an embedded linux using buildroot. after generating 
rootfs.tar file system, I have tested the embedded linux and figured out 
that every change I made to the linux is not taking place after reboot. 

my question is: how to enable buildroot linux to have a persistence file 
system and can save changes? 

I have generated: rootfs.tar , rootfs.cpio , bzImage of kernel. 

any help appreciated. 
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