dtbtool: add new --dt_tag switch

* Add new switch named --dt_tag (or -d)
* Allows a custom QCDT_DT_TAG
* If --dt_tag/-d is not used, it defaults to "qcom,msm-id =<" as it is today.

This should allow more flexibility with devices requiring various DT_TAGs going forward.

Change-Id: I6f6a9a55b3b94ce6103a4890b28792380fe1e274