Support GKI boot.img v4 signing

Commit I9967d06bde0e18a12b84b5b0b568db09765fe305 supports adding a
generic boot_signature into boot.img v4. This change allows replacing
the boot_signture signing key with a release key during the release
process.

The default GKI signing key can be specified in a BoardConfig.mk via:

  BOARD_GKI_SIGNING_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem
  BOARD_GKI_SIGNING_ALGORITHM := SHA256_RSA2048
  BOARD_GKI_SIGNING_SIGNATURE_ARGS := --prop foo:bar

The release signing key/algorithm can be specified by the following options
when invoking sign_target_files_apks:

  --gki_signing_key=external/avb/test/data/testkey_rsa4096.pem
  --gki_signing_algorithm=SHA256_RSA4096

Additional arguments for generating the GKI signature can be
specified as below:

  --gki_signing_extra_args="--prop gki:prop1 --prop gki:prop2"

Bug: 177862434
Test: make dist
Test: sign_target_files_apks \
        --gki_signing_key=external/avb/test/data/testkey_rsa4096.pem \
        --gki_signing_algorithm=SHA256_RSA4096 \
        --gki_signing_extra_args="--prop gki:prop1 --prop gki:prop2" \
        ./out/dist/*-target_files-eng.*.zip signed.zip
Test: Checks GKI boot_signature is expected after signing:
      `unzip signed.zip IMAGES/boot.img`
      `unpack_bootimg --boot_img IMAGES/boot.img --out unpack`
      `avbtool info_image --image unpack/boot_signature`
Test: unit test: releasetools_test and releasetools_py3_test

Change-Id: I61dadbc242360e4cab3dc70295931b4a5b9422a9