only the upper layers of the Android stack
This is *perhaps* not entirely true. Meizu have their Ubuntu pre-loaded MX4, and Ubuntu ROM for existing MX4 users coming in December. It's possible that Mediatek has produced native drivers for Linux, rather than using their existing Android ones and running it through libhybris (which Ubuntu also uses). This is a very high volume device from a big OEM. It's not beyond the realms of possibility that drivers for the main SoC will be native (though I'm sure everything else will be Android).