kernel: Allow passing empty DTB to mkbootimg via board-flag

* Boot Image header v2 demands a DTB be passed by default.
* We don't /need/ one at all, as we have and use a dedicated
  DTB partition.
* Signing hates trying to find dtb.img anywhere for some magical
  reason, so cheat and introduce a flag to include a blank one.

Change-Id: I889ce3815476a55829870b30de1b9210283a79ae