mount-encrypted: add encrypted mount helper

This adds the utility needed to mount the encrypted partition at boot
time, as defined by the design document:
https://docs.google.com/a/google.com/document/d/1VQTDXvNsEFcrUOmNC4OmCfJst49Pd_mxZ41nfKu5EPc/edit
This still needs Cryptohome support and chromeos_startup to switch to
using it.

BUG=chromium-os:22172
TEST=lumpy build & manual testing

Change-Id: Ib9f0b4e5ba1a8aeb4737d0c8ec72a8e0dee049da
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/16889
Reviewed-by: Elly Jones <ellyjones@chromium.org>
6 files changed