hack(cros_generate_update_payload): Work around broken gpt handling

The old legacy_offset_size_export and dependent code only works with the
pre-usr disk layout. To get things a bit less broken just hard-code the
partition number (3) for now. Most of this script can be deleted but it
is late and I need this to work...
hack(devserver): Gut 'stateful payload' related code.

Updating the stateful partition always sounded like a terrible idea.
fix(devserver): Respect the --board argument

The update server implementation was always using the default board
regardless of what the user provided on the command line.
Merge pull request #8 from marineam/usr

Updates for /usr images