mkdtimg: reuse identical FDT blobs in dtbo image

When multiple dt_table_entries uses the same FDT, do
not keep multiple copies in the output dtbo partition
image. Instead, keep dt_offset in the dt_table_entries
point to the same blob.

Update mkdtimg_testdata.sh to test the above case too.

Bug: 36792954
Test: run mkdtimg_testdata.sh
Change-Id: I3413b52e1174154d9f6146efd8cead3f50c0f256