Change key derivation to scrypt

Change key derivation to scrypt

scrypt is a sequential memory-hard key derivation algorithm that makes
it more difficult for adversaries to brute force passwords using
specialized equipment. See http://www.tarsnap.com/scrypt/scrypt.pdf for
more details of the algorithm.

This adds support for initializing disk encryption using scrypt and
upgrading from the previous PBKDF2 algorithm.

Change-Id: I1d26db4eb9d27fea7310be3e49c8e6219e6d2c3b