Switch to using PBKDF2 for the key generation for OBBs. Any previously
generated OBBs will stop being read correctly. A small pbkdf2gen program
is available to allow generation of appropriate keys with the salts.
Bug: 3059950
Change-Id: If4305c989fd692fd1150eb270dbf751e09c37295
* 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
