Update Atreus to current code conventions (#5849)

* Update atreus to current code conventions - add multi revision instead of defines

* Remove config.h duplication from user keymaps

* Add breaking change log

* Add missing pragma once

docs/ChangeLog/20190830/PR5849.md

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+*  Update Atreus to current code conventions
+    * Duplicate include guards have bypassed the expected header processing behavior
+    * All keymaps affected are recommended to remove duplication of `<keyboard>/config.h` to `<keyboard>/keymaps/<user>/config.h` and only provide overrides at the keymap level