Allow path to the cgpt binary to be set on the command line

BUG=chromium-os:17138
TEST=tested changes on vm8-m2, was able to successfully run au-generate.py
     and it used the cgpt binary from au-generate.zip

Change-Id: Ia57f1be4b0d669cad430e51977cce6e26d704320
Reviewed-on: http://gerrit.chromium.org/gerrit/7796
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Reviewed-by: Eric Blake <eblake@chromium.org>
Tested-by: Eric Blake <eblake@chromium.org>
(cherry picked from commit 236faae91a224b36fb48ec407535a426f1f67877)
Reviewed-on: http://gerrit.chromium.org/gerrit/8061
Commit-Ready: Eric Blake <eblake@chromium.org>
2 files changed