SDM845 generic device support

For an installation procedure and background info see the wiki page https://wiki.postmarketos.org/wiki/Qualcomm_SDM845_(generic-sdm845)

Basically, we flash U-Boot to the boot partition and run it instead of Linux. It then provides us with a nice EFI environment and handles DTB selection automatically.

The U-Boot build I have tested this with can be found here. Press the power button during the autoboot delay to access a menu with some useful features including mass storage mode, U-Boot console serial gadget, and a very broken fastboot implementation.

I have tested migrating an existing FDE installation on a OnePlus 6 to the generic device support and documented the process on the wiki.

Issues:

• Firmware conflicts (board-2.bin needs to get upstreamed or we need a generic one for just our devices, BT firmware requires kernel changes to support specifying a custom path)