Add "obbtool" host command for adding, removing, and querying Opaque
Binary Blob (OBB) information from a file.
Change-Id: Id2ac41e687ad2a500c362616d6738a8ae7e8f5c3
* Move error messages around to clarify the errors.
* Add extra error check when reading a file.
* Seek to the end of a file when writing the signature so the users of
the API don't have to remember to do it.
Change-Id: I2337051b9f9fa8147c5900237deec790dcd92436