releasetools: Make scripts pylint clean.

Clean up the following scripts.
  blockimgdiff.py
  common.py
  edify_generator.py
  img_from_target_files.py
  ota_from_target_files.py

Seems we don't have a way to force pylint-ing the scripts using
per-directory pylintrc files (and we don't have pylint tool in AOSP
repo), per
https://android.googlesource.com/platform/tools/repohooks/#todo_limitations.

Test: `m dist`
Test: pylint --rcfile=pylintrc <script.py>
Change-Id: Ia6fd1ddc86f4d84c68e500f225d4a89d0fea8ec7