Update thirdparty CMakeLists for TensorFlow 2.18.1

The TensorFlow Lite build system (v2.18.1) requires the
TFLITE_HOST_TOOLS_DIR CMake variable to be set or else the CMake
configuration step fails. The build system will use the tools found in
PATH _before_ looking in whatever folder is specified in this variable,
but the variable is not allowed to be empty. If you don't have the
required host tools (currently flatc only) in your PATH, make sure to
set this variable to the correct folder where the tools can be found.

Silence deprecation warnings for third party libraries and/or their
dependencies. TensorFlow Lite downloads its own dependencies, but the
regular cmake_policy calls will not affect these, hence setting the
variables for policy CMP0169 instead.

Change-Id: I99983e757efbd6cfbce6c4a3c368db02413bb0d8
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>