releasetools: Properly handle boot images that use dt

Instead of using a prebuilt boot/recovery image, bring the dt image
into the target files package and use it during package creation

Change-Id: I70ecf74423aad95a3eb8e085029c37450ae354a2

Conflicts:

	tools/releasetools/common.py
releasetools: Fix whitespace error for INSTALLED_DTIMAGE_TARGET

Copy/paste produced spaces instead of a tab; fix that

Change-Id: I937e5f6ffd2c947c77dc4baa7427b170c1ea4bcd