fix(cros_generate_update_payload): Remove duplicate chromeos-common.sh

An extra source of chromeos-common.sh was added, fancier search logic
for that file already existed a couple lines down and that logic is
required for au-generator.zip
feat(cros_generate_update_payload): Add features from core_upload_update

Add the metadata generation and signature verification steps from
core_upload_update to keep similar operations together.
Merge pull request #1 from marineam/updates

feat(cros_generate_update_payload): Add features from core_upload_update