make full OTAs block based

Instead of writing individual files and fixing up their metadata, make
full OTAs contain a system image and simply write it to the block
device.

This is only done for target-files that already contain the recovery
flashing information, older target-files still get a file-based full
OTA.

Bug: 12893978
Change-Id: If7586083c8f275e24fec49d260af5b5aff4a0a88