pine64-pinephonepro: switch to UEFI booting

This removes u-boot from our image and allows booting via an UEFI-compatible platform firmware instead. So to flash pmOS on the PPP now we will require the user to flash a platform firmware image once after which they can write this pmOS image to an sdcard or the internal storage

I recommend using https://github.com/Tow-Boot/Tow-Boot as the platform firmware of choice. Once flashed to internal storage, hold the volume up button while booting to boot into UMS mode. Plug your phone into a PC and flash to the storage that appears. You can also flash to an sdcard instead and hold volume down while booting to boot from it rather than the internal storage