scripts: Add support for unibuild json format

This script was calling fdtget directly on the dtb file, so I didn't
find it from the migration.  Changing to check the json file if it's
present instead.

BUG=chromium:840969
TEST=built and tested soraka image

Change-Id: Ib0e999d91f67e5e3aa657e30d8bac410a77f2aef
Reviewed-on: https://chromium-review.googlesource.com/1056092
Commit-Ready: C Shapiro <shapiroc@google.com>
Tested-by: C Shapiro <shapiroc@google.com>
Reviewed-by: C Shapiro <shapiroc@google.com>
1 file changed