Wltests pixel2 support

Created by: derkling

This series refactor a bit test_series and extends it to support Pixel 2 devices. Thanks to @credp for the identification of a stable sequence to flash update a Pixel device with a new kernel and modules.

The first couple of patches progress also submodules dependencies, although they can go into a different PR I've kept them here just because that's the most advanced stack I've used to test the integration on pixel devices.