generate_delta_main: add out_metadata flag
generate_delta_main: add out_metadata flag

out_metadata lets us grab the metadata size and metadata rsa signature
for a payload:

delta_generator -out_metadata /tmp/update.metadata -private_key update.key -in_file /tmp/update.gz

mjg59/update_engine,endocode/update_engine,marineam/update_engine,marineam/update_engine,endocode/update_engine,mjg59/update_engine,marineam/update_engine,mjg59/update_engine,endocode/update_engine,marineam/update_engine