Add logic to generate boot image mainline extension.

This CL only adds the logic but does not enable it because the check
for boot image mainline extension is not ready. After this change, the
logic is only exercised in tests.

There will be a follow-up CL to enable it.

Bug: 269230245
Test: atest art_standalone_odrefresh_tests
Change-Id: Ia39733f4a0ea7ff17c0a9fab33faf09e7f5d2a87