{ros2} Add required ROS2 SDK environment variables

[sgstreet]

This PR adds the following environment variable when the SDK is setup:

PYTHON_SOABI required to avoid setup failures with python packages
PYTHON3_NUMPY_INCLUDE_DIR required to using python opencv binding
PYTHONWARNINGS to suppress setuptool warmings
AMENT_SKIP_SHELL_PATH to prevent ament from prepending $OECORE_TARGET_SYSROOT/opt/ros/$ROS_DISTRO/bin to glocal PATH variable
PYTHONPATH to include the target site-packages to the python environment

If the Yocto enviroment variable ROS_SDK_UNIFY is not empty, $OECORE_TARGET_SYSROOT/opt/ros/$ROS_DISTRO/setup.bash or friends is source during SDK setup. The value of ROS_SDK_UNIFY should one of the following "sh", "bash" or "zsh"

[sgstreet]

Still verifying this on master-next and styhead-next

[sgstreet]

@robwoolley I verify this by building examples, demos and example-interfaces with master-next and styhead-next. We still have the issue of PYTHON_SOABI verses _PYTHON_SYSCONFIGDATA_NAME. I have been unable to make the latter work.

[sgstreet]

I have made a mess out of this, fixing now.