dtbtool: Add qcom,board-id support (version 2)

Add support to parse the new split qcom,msm-id/qcom,board-id
in DTB files.  The original qcom,msm-id with soc_id/platform/
soc_rev is split into qcom,msm-id with only soc_id/soc_rev
and a new qcom,board-id with platform/subtype.

The tool always generate the new table of device tree
format (v2), but is backward compatible with parsing original
DTB files with only qcom,msm-id (in which case subtype is
assumed zero).

This change requires a corresponding bootloader change to
properly parse the updated table of device tree image
format (v2).

Change-Id: Iee4a0bf7de026ffc7f610ad485254c4b192ed3eb