XZ2 support for wltests
Created by: valschneider
Tested locally, had to add some intrusive changes to have something working and not spend a whole week on it.
DTBO generation is currently broken on the kernel (https://github.com/sonyxperiadev/kernel-copyleft/tree/51.1.A.2.xxx), and even after getting the DTBO files flashing them seems to soft-brick the phone. This only flashes boot.img for now.