Adjust the order in which image-specific `sysbuild.cmake` files are
iteratively included by sysbuild.
This is motivated by the introduction of `sysbuild_add_dependencies()`.
In the following example:
sysbuild_add_dependencies(CONFIGURE my_sample sample_a sample_b)
the `my_sample` image is expected to be added before this function is
called. Success depends not only on the placement of the call, but on
the order in which new images are added, which itself is influenced by
the order in which `sysbuild.cmake` files are included. This last order
can be tweaked to make the "dependencies" feature more user-friendly.
This is done by rewriting the internal `sysbuild.cmake` processing loop
into a new, general purpose function - `sysbuild_add_subdirectory()` -
which is a wrapper for `add_subdirectory(<source_dir>)` that recursively
includes `sysbuild.cmake` files for all images found in `<source_dir>`.
With the new function, all images that are expected to be found in a
given `<source_dir>` are guaranteed to be added around the same time.
This wasn't the case with the old processing loop, because the image-
specific `sysbuild.cmake` files (where "sub-images" could be defined)
were left to be processed at the very end.
Below is the initial order in which sysbuild will add all images.
Note: the order of Zephyr modules (from 1 to n) is well-defined.
1. Main application (aka DEFAULT_IMAGE)
2. MCUboot (optional)
3. All images added via these directories:
3.1. <module-1>.sysbuild-cmake
3.2. <module-2>.sysbuild-cmake
...
3.n. <module-n>.sysbuild-cmake
4. All images added via these files:
4.1. ${BOARD_DIR}/sysbuild.cmake
4.2. ${APP_DIR}/sysbuild.cmake (aka sub-images of DEFAULT_IMAGE)
These images are intended to be sorted for the users' convenience, from
most to least important in the build system, or least to most dependent
on other images for configuration (potentially).
Finally, the use of `sysbuild_add_subdirectory()` requires updating the
directory structure in sysbuild:
./images
- All images should belong here. The `DEFAULT_IMAGE` should be the
first and only image at the top level, so that it gets added first
and its sub-images get added last.
./images/bootloader
- Moved from ./bootloader.
./images/boards
- Adds images from the board-specific `sysbuild.cmake` file.
Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>
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