releasetools: Support validating Verified Boot images.

For a given (signed) target-files.zip, this CLs allows verifying the
Verified Boot related images. It works with both of VB 1.0 and VB 2.0
images.

As part of the CL, it also moves validate_target_files.py to argparse,
which is more flexible than the traditional getopt module.

Also add unittests for the VB 1.0 path. VB 2.0 tests will be added in
follow-up CL.

Example usage:

- Run the script on aosp_bullhead target-files.zip.

$ ./build/make/tools/releasetools/validate_target_files.py \
    --verity_key build/target/product/security/verity.x509.pem \
    --verity_key_mincrypt build/target/product/security/verity_key \
    aosp_bullhead-target_files-4522605.zip

- Run the script on aosp_walleye target-files.zip.

$ ./build/make/tools/releasetools/validate_target_files.py \
    --verity_key external/avb/test/data/testkey_rsa4096.pem \
    aosp_walleye-target_files-4627254.zip

Bug: 63706333
Bug: 65486807
Test: Run validate_target_files.py on target_files.zip files.
Test: PYTHONPATH=build/make/tools/releasetools python -m unittest \
          test_validate_target_files
Change-Id: I170f14d5828d15f3687d8af0a89a816968069057