merge_dtbs: Use logging and argparse

Use the logging module to log important information to various log
levels: info, debug, warning and error.
- Modify __str__ representations of classes DeviceTree and
  InnerMergedDeviceTree for readability in debug logs
- Make parse_dt_files() return instead of yield so as to order all the
  filename parsing at one go after the preceding log messages

Use the argparse module to specify script arguments cleanly. Also make
the corresponding change to merge_dtbs.sh to invoke this script with the
correct flags.

Change-Id: Id09d9d7b34cde60eff99876f0c0c4f6643852aee
Signed-off-by: Guru Das Srinagesh <quic_gurus@quicinc.com>