Enable phone encryption.

Phone encryption (which can be enabled in the GUI through
Settings -> Security -> Encrypt phone) requires a partition
to be marked as encryptable. Otherwise the following error
is thrown:

  D/VoldCmdListener(  137): cryptfs enablecrypto inplace {}
  E/Cryptfs (  137): Cannot get size of block device

Set encryptable flag on /data, and store encryption keys
in the footer (the last 16KB of the partition).