Deprecate mkbootimg_args.json by fully support "--format=mkbootimg"

* Fully support "unpack_bootimg --format=mkbootimg" to output boot /
  vendor_boot repack commands.
* Deprecate mkbootimg_args.json, as they serve the same purpose.
* Refactor unpack_bootimg and mkbootimg_test.
* Migrate repack_bootimg to use output of "--format=mkbootimg" instead
  of mkbootimg_args.json.

Bug: 174443192
Test: atest --host mkbootimg_test
Test: ./repack_bootimg.py --src_bootimg <> --dst_bootimg <>
Change-Id: I0545d1d1d83d70fe24339c09e58f6b0225dfe62e