dtbhtool: Add LOCAL_MODULE_TAGS := optional

* This breaks building on devices that
  don't require the need of dtbhtool
* Also remove LOCAL_MODULE_TAGS := eng, to
  avoid requiring this on an eng build

Change-Id: Iad9296f46dd78c778ce1ce48d6722ed98bc27cd3
Signed-off-by: Paul Keith <javelinanddart@gmail.com>