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
Merge remote-tracking branch 'cm/cm-11.0' into cm-11.0