Renamed avbtool to avbtool.py so the repo upload hook for pylint
actually picks up changes to the avbtool.py. Also put in a symlink to
point from avbtool to avbtool.py to keep compatibility with
libavb_host_unittest.

This change is still compatible with Python 2.

Bug: 154319654
Test: external/avb$ atest :all
Test: ./aftltool_integration_test.py
Change-Id: Icb53a333e1e5ea1c54b6e4694fc6794f2223c94b