Use env python

Change the shebang from /usr/bin/python to /usr/bin/env python
Some users, especially on older systems don't use the python
supplied with the OS. This has an impact with module loading if
the python in /usr/bin is 2.6, and user wants to use 2.7
in /usr/local/bin, for example

Change-Id: Ib31b369a94c33fb7508f0539c2b2f14177e507e0
Rework package-installs to collapse on build host

Instead of doing the work in the image of parsing through the element's
package-install declarations, we can squash it on hostside, where we
have both YAML and JSON available to us, and then emit a single
pre-processed file into the target to be used later.

Change-Id: I3f182aa3aae0a79b2f3ea4e66c1878ad12878b0a
Merge "Use env python"