Avb sign boot images

The bootimg module had a very different implementation of avb signing
than what was in make. Add an alternate implemenation that is used
when avb_private_key is not set.

Bug: 377562951
Test: m nothing --no-skip-soong-tests
Change-Id: I253bcc8135f3d294eb6e09f39429c84c7c0fc037