releasetools: Fix whitespace error for INSTALLED_DTIMAGE_TARGET

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

Change-Id: I937e5f6ffd2c947c77dc4baa7427b170c1ea4bcd
releasetools: bring back boot.img handling

This is required for galaxysmtd and other devices with custom
boot format.

This partially reverts "releasetools: Properly handle boot images that use dt"

Change-Id: Ib35001c2d9b906067e5a3c84712dc3a283a38cbb