[nrf fromtree] doc: document CMAKE_BUILD_PARALLEL_LEVEL and VERBOSE env variables

Sysbuild may include several images and when using ninja as build tools
then build options like `ninja -v` or `ninja -j <n>` is only consumed
by the top-level build folder, which is sysbuild itself.

CMake supports setting number of jobs or verbosity using environment
variables.

Document this feature in the Building with sysbuild section.

Signed-off-by: Torsten Rasmussen <[email protected]>
(cherry picked from commit e270fd4)

Author: tejlmand

doc/build/sysbuild/index.rst

@@ -143,6 +143,25 @@ As mentioned above, you can run sysbuild via ``west build`` or ``cmake``.
       an extra CMake argument. ``APP_DIR`` is the path to the main Zephyr
       application managed by sysbuild.
 
+      .. tip::
+
+         The environment variables, ``CMAKE_BUILD_PARALLEL_LEVEL`` and ``VERBOSE``, can be used to
+         control the build process when using sysbuild with CMake and ninja.
+
+         To set number of jobs for ninja for all sysbuild images, set the CMAKE_BUILD_PARALLEL_LEVEL
+         environment variable and invoke the build with ``cmake --build``, for example:
+
+         .. code-block:: shell
+
+            CMAKE_BUILD_PARALLEL_LEVEL=<n> cmake --build .
+
+         For verbose output of all images, use:
+
+         .. code-block:: shell
+
+            VERBOSE=1 cmake --build .
+
+
 Configuration namespacing
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