enable EFI on sdm845 phones

Enable support for generating systemd-boot configs on SDM845 devices while continuing to support the Android boot image.

This additionally switches over the flash method to fastboot-bootpart, so the pmOS boot and root partitions are exported seperately. The expectation is that users STOP using the subpartitioning scheme and flash the two partitions separately.

This will enable a smooth transition to U-Boot by simply flashing it to the boot partition.

The new default configuration here is:

	OnePlus 6	OnePlus 6T	SHIFT6mq
Boot	systemd	system	super
Root	userdata	userdata	userdata

TODO

• This may need adjustments in BPO to export the boot image as well as the boot/root partitions.
• We will also need to update the wiki pages to explain that the pmOS boot and root partitions are flashed to separate partitions on the phone.
• Merge boot-deploy!68 (merged) and make a new release