releasetools: Store and use the dt image file through target files

Target files packages may be used for signing images separate from the
build process. Store the device tree image file in the target files
package so it can be used during the signing process.

Change-Id: Ie8507121fa9c4ba57ecffeab05bd859ae5f5b788
Ticket: CYNGNOS-1744
(cherry picked from commit a3219276)