certify_bootimg: support --gki_info for a boot.img

The gki_info file can be used to append additional
'extra_args' during the certification process.

An usage example:
  certify_bootimg --boot_img_archive boot-img.tar.gz \
      --algorithm SHA256_RSA4096 \
      --key external/avb/test/data/testkey_rsa4096.pem \
      --extra_args "--prop foo:bar" \
      --gki_info /path/to/gki-info.txt \
      --output boot-certified-img.tar.gz

An example of the file content of the gki-info.txt:
  certify_bootimg_extra_args=--prop KERNEL_RELEASE:5.10.107-android13-1-00361-gf1e8564c5530-ab8332003

Bug: 230426945
Test: atest --host certify_bootimg_test
Change-Id: I484f77b4fd6eacb3cdfc2270543a2cd6d33d58c1
2 files changed