cmake toolchain file improvements

[shuhaowu]

CMake toolchain needs set(PYTHON_SOABI cpython-38-aarch64-linux-gnu) to enable for cross-compilation for Python ROS. This is kind of problematic to implement because the variable must be set to the target Python version. Right now the toolchain file is global for the whole builder.

This requires a bit more thought. Ideally the ros-rt-img tool will generate a cmake toolchain file dynamically. I'll have to think about it a bit more.