Kconfig: Have CMake print a note about the changed 'default' behavior

Always print the note to make sure it isn't missed, but make it easy to
disable (by creating an empty file).

The note will be removed later.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>

.gitignore

@@ -40,3 +40,6 @@ tags
 .xxproject
 .envrc
 .vscode
+
+# Disables the warning about the changed behavior for Kconfig defaults
+hide-defaults-note

cmake/app/boilerplate.cmake

@@ -322,3 +322,20 @@ foreach(boilerplate_lib ${ZEPHYR_INTERFACE_LIBS_PROPERTY})
     ${boilerplate_lib}
     )
 endforeach()
+
+
+if(NOT EXISTS ${ZEPHYR_BASE}/hide-defaults-note)
+    message(STATUS "\n\
+*******************************\n\
+*** NOTE TO KCONFIG AUTHORS ***\n\
+*******************************\n\
+\n\
+The behavior of Kconfig 'default' properties in Zephyr has changed. The \n\
+earliest default with a satisfied condition is now used, instead of the \n\
+last one. This is standard Kconfig behavior.\n\
+\n\
+See http://docs.zephyrproject.org/porting/board_porting.html#old-zephyr-kconfig-behavior-for-defaults.\n\
+\n\
+To get rid of this note, create a file called 'hide-defaults-note' in the \n\
+Zephyr root directory. An empty file is fine.")
+endif()