In device specification: allow devices to opt out of full (non-diff) upgrades

[floitsch]

When a device needs to upgrade, it has the option of falling back to a non-diff-based update where it downloads the whole new firmware. For devices that are behind a costly connection (nb-iot) this might not be desirable.

We should add an option to the device specification that declares that a device should never use the full firmware for upgrading.

Note that this situation (not having any patches) should be quite rare. Especially, if the broker is stable.