sign channel metadata to enforce release channel name

[thestinger]

After the zip is verified, the metadata is verified against it to make sure that it was accurate. However, it would be nice to have offline signing of the update channel metadata in a way that enforces the channel name. At the moment, if an attacker takes over the server, they can't do much, but one thing they could do is move the current beta release into the stable channel.

[thestinger]

The channel name is now in the metadata but this probably should have included the device name too so we'll need to add another field with the device name. Essentially, it needs to have the file name (DEVICE-CHANNEL) in the metadata.