The postinst behaviour needs to be verified, specifically that a postinstall that should run on the host does, and a postinstall that should run on first boot also does.

One idea for implementing this would be a package that creates a file in postinst at rootfs time, and then inspect the contents of the image before booting (either extracting the filesystem, or checking the build dir) to verify it exists.  Another recipe would create a file in the postinst on the target, verify that it doesn't exist on the host and then that it does exist once it's been booted.