Add build-level tests verifying configdep selective rebuild behavior:
- test_configdep.py: selective rebuild (only affected .obj rebuilt) and
ELF string verification after Kconfig toggle
- test_rebuild_configdep.py: full rebuild graph tests with configdep enabled,
including the two-build "settle" pattern for .cdep stub timestamps
Place the configdep test app in tools/test_build_system/configdep_test_app/
alongside the build system test infrastructure. Split configdep-specific
rebuild coverage from pure CMake/Ninja rebuild tests (test_rebuild.py runs
with configdep disabled via a module-local idf_py fixture).
