Allow images to be readded to target-files

There may be cases where various partitions were
modified (such as extra signing of boot/recovery)
which require the system partition to be regenerated
with a new recovery patch script.  Allow a request
for the recovery patch to be rebuilt, and for
missing images to be readded to the existing images.

Change-Id: Ie93c5bc1161a0d5002002dca23e07742ce2ba468