Support automatic kernel updates on Android devices

Depends on !2214 (merged).

This MR adds all the bits needed for automatic kernel updates for Android devices! It also enables that functionality on the OnePlus 6 and 6T.

I've added a new deviceinfo variable: deviceinfo_flash_kernel_on_update, the kernel will not be flashed unless this variable is set. There is also a new function in mkinitfs_functions to run pmos-update-kernel if it is installed. It's not installed automatically and should instead be depended on in the device APKBUILD.