Add support for v2 of dm-default-key

Version 2 of dm-default-key has an extra parameter and always sets the
DUN.

Bug: 147814592
Test: Cuttlefish boots with keydirectory flag
Test: Crosshatch formatted before this change boots after it
Change-Id: I59081e385324d2e34a5f252286a97938d6ffb79b