idf.py: Disable CMake --warn-uninitialized option by default

Can still be enabled by passing --cmake-warn-uninitialized on the command line

Prevents CMake warnings printed by default if IDF_PATH is underneath the CMake
project directory.

The reason for this is that CMake --warn-uninitialized only enables checks
inside the project directory (ie top-level CMakeLists.txt directory and
subdirectories), it doesn't enable for files included from other directories.

(The only way to enable warnings in other directories is to pass
--check-system-dirs and this looks like it's only useful for CMake's own
developers as it prints a lot of warnings from inside CMake otherwise - see
https://gitlab.kitware.com/cmake/cmake/-/issues/19645 )

Plan to follow up with a later commit to clean up most of the warnings (which
aren't problems for CMake execution), but we'll also disable this option by
default to avoid this unexpected triggering of IDF warnings.

tools/idf_py_actions/tools.py

@@ -200,7 +200,7 @@ def ensure_build_directory(args, prog_name, always_run_cmake=False):
                 "-DPYTHON_DEPS_CHECKED=1",
                 "-DESP_PLATFORM=1",
             ]
-            if not args.no_warnings:
+            if args.cmake_warn_uninitialized:
                 cmake_args += ["--warn-uninitialized"]
 
             if args.define_cache_entry: