Create and use a salt when calling pbkdf2 to encrypt/decrypt the master key.

Create and use a salt when calling pbkdf2 to encrypt/decrypt the master key.

In order to prevent rainbow table attacks on decrypting the master key,
create a 16 byte "salt" by reading /dev/urandom.  This is done right after
reading urandom to get the master key for the filesystem.  The salt is
stored 32 bytes after the end of the key (a padding added to help prevent
accidental overwriting of the salt) and the salt is fixed at 16 bytes long.

This change will make existing encrypted filesystems unusable.

Change-Id: I420549d064c61d38aea78eef4d86c88acb265ca3