build: use /usr/bin/env to call python

Hardcoding the path to python instead of env is not recommended.  This
can break the build in some Linux environments that link /usr/bin/python
to python3.

Change-Id: I4c62eb391acc6d468ce24ac8640834e965e76e78
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