sysbuild: create intermediate file for CMakeCache.txt configuration

Using `configure_file()` directly on an image's CMakeCache.txt file
creates a sysbuild CMake configure dependency on the image's cache file.

This means that any change to the CMake cache on any image results in a
CMake re-run on all images.

Create an intermediate file for configure_file() to depend upon.
The aligns sysbuild in nRF Connect SDK behavior with Zephyr wrt. image
CMake cache dependencies.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

sysbuild/CMakeLists.txt

@@ -25,7 +25,12 @@ function(configure_cache)
 
   ExternalProject_Get_Property(${CONFIGURE_CACHE_IMAGE} BINARY_DIR)
 
-  configure_file("${BINARY_DIR}/CMakeCache.txt" "${BINARY_DIR}/CMakeCache.txt.tmp" @ONLY)
+  # Copy the cache file to avoid a dependency to the image's cache file.
+  # A dependency on the image's cache file cause unneccessary CMake re-runs for
+  # all sysbuild images whenever a setting in the cache changes, even for not
+  # sysbuild relevant changes.
+  zephyr_file_copy("${BINARY_DIR}/CMakeCache.txt" "${BINARY_DIR}/CMakeCache.tmp" ONLY_IF_DIFFERENT)
+  configure_file("${BINARY_DIR}/CMakeCache.tmp" "${BINARY_DIR}/CMakeCache.txt.tmp" @ONLY)
   zephyr_file_copy("${BINARY_DIR}/CMakeCache.txt.tmp" "${BINARY_DIR}/CMakeCache.txt" ONLY_IF_DIFFERENT)
 endfunction()