Add capability to compress dt overlays

For DTBOs using a header of version 1, mkdtboimg.py
supports zlib/gzip compression as per the
compression format information present in the
4 least significant bits of the flag
field in dt_table_entry.

A new argument "decompress" has been added to the
"dump" command to decompress any compressed
entries before dumping.

Bug: 109892148
Test: sh mkdtboimg_testdata.sh
Test: make vts -j32 && vts-tradefed run vts -m
      VtsFirmwareDtboVerification
Change-Id: Id9524f986506ee8eb91d0ed15d4b91a2d2ed85af