Update generate_au_zip.py location for cgpt.

Cgpt was moved and a symlink based wrapper was added. That wrapper will
be improved soon, when when that's true we'll need to change this back.

A specific note... cgpt is currently statically linked. If that wrapper does
not remain statically linked, then a simple revert won't be enough.

BUG=chromium-os:39814
TEST=Manual au-generate.zip creation.

Change-Id: I2705b1eddd8ef28c7eb099512513daf80f586218
Reviewed-on: https://gerrit.chromium.org/gerrit/45128
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
fix(generate_au_zip): Remove unused scripts from au-generator.zip

We have no need to ship convert_recovery_to_ssd.sh with builds.
fix(common.sh): Fallback to loading version.txt from current directory.

This makes version handling continue to work normally with
au-generator.zip
fix(core_upload_update): Add to au-generator.zip, move some operations.

Add core_upload_update to au-generator.zip which requires some extra
logic to make it runnable anywhere it may be. To organize the code a
little better all the delta_generator calls have been moved to
cros_generate_update_payload. core_upload_update is now just a wrapper
around cros_generate_update_payload and core-admin.
fix(core_upload_update): Set PATH for au-generator.zip bundles.

The tools provided by au-generator.zip are expected to be in the PATH by
this script and the scripts it calls. Make it so.
fix(generate_au_zip): Add core-admin to au-generator.zip

Forgot to add this in previous commits, so here it is!
Merge pull request #28 from marineam/updates

Updates for au-generator.zip