Rework packages caching mechanism

Packages are now built by kokoro and pulled in to a local build
unless FETCH_PACKAGES=false . All packages are copied into
rootfs.img in the form of a local debian repo so a user can install
e.g. -dev packages later.

To build packages locally:
rm -rf cache/packages.tgz out

In this change FETCH_PACKAGES actually defaults to false. This will be
changed when the first kokoro artifact is available.
PACKAGES_FETCH_ROOT_DIRECTORY is primed with a packages.tgz for testing

Change-Id: I626587fda2b963183066c2a81223b047afa514eb
15 files changed