The `idf_as_lib` example is used and tested in
`tools/test_build_system/test_cmake.py`. Include `idf_as_lib` in the
build_system rules to ensure the tests are executed whenever there is a
modification in the `idf_as_lib` example.
Expand the `test_build_custom_cmake_project` test to cover all supported
targets, including host build.
Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>