python3native, pythonnative: export PYTHON_LIBRARY and PYTHON_INCLUDE_DIR

python3native, pythonnative: export PYTHON_LIBRARY and PYTHON_INCLUDE_DIR

packages can use

find_package(PythonInterp REQUIRED)
find_package(PythonLibs REQUIRED)

while we control PYTHON pointing to native py3 the libs and include
directories will then point to build host version, which can result in
unexpected combination and if we are lucky we get errors if its quite
different e.g. py2 libs/includes and py3 executable

This variable can be then used to export PYTHON_LIBRARY and
PYTHON_INCLUDE_DIR so that above find_packages can work correctly

see [1] for how it happens in cmake

LLDB uses it see [2]

[1] https://github.com/Kitware/CMake/blob/master/Modules/FindPythonLibs.cmake
[2] https://github.com/llvm/llvm-project/blob/master/lldb/cmake/modules/LLDBConfig.cmake#L226

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>