sync etna driver to cc7cbb2
native_fbdev_driver: allow choosing between drivers

Choose between drivers with environment variable EGL_FBDEV_DRIVER,
current choices are etna or swrast.
This still needs some form of probing, currently the default is etna.
Although swrast is very slow on ARM, this is useful for comparing the
correctness of the output.
sync etna driver to d586740