docs: release-notes: Add note on fixed zephyr build system usage

Adds notes of these changes

Signed-off-by: Jamie McCrae <[email protected]>

Author: thedjnK

docs/release-notes.d/zephyr-folder-paths.md

@@ -0,0 +1,10 @@
+- Zephyr signature and encryption key file path handling has now
+  been aligned with Zephyr, this means values can be specified in
+  multiple .conf file and the one that last set it will be the set
+  value. This also means that key files will no longer be found
+  relative to the .conf file and will instead be found relative
+  to the build system ``APPLICATION_CONFIG_DIR`` variable, though
+  the key file strings are now configured which allows for using
+  escaped CMake variables to locate the files, for example with
+  ``\${CMAKE_CURRENT_LIST_DIR}`` to specify a file relative to
+  the folder that the file is in.