My last try for the day: What I've found (you can ignore all this and just try the one file linked to in post 30) is that the same named files exist all over the tree, and that's fine and it makes sense to me, but not being used to cm11 I didn't know for certain which one's were good for my purpose.
My last try which is in the test-* image now, was from the files_context file in the ./device tree. It "appeared" to work for me on a backup and a restore, but I'm not 100% sure I believe it due to the same dates and times being on the files. On the other hand, tar can easily do that, so it maybe is good now.
Good luck
Depends on the flag:
+= will merge to core se context
= will only use files from device
So better add it in device tree if it has such a file
You are not risking a lot. The format actually is failing. If you can format, the restore will go on