Internal checksums are part of the wider topic of image meta data, which is available for modern build recipes - but not legacy ones (yes, all devices 'should' be converted over, but given the amount legacy devices that's probably not going to happen anytime soon or ever, especially for less popular devices; the migration from ar71xx to ath79 takes care of a lot of them, so will the retirement of the ath25 target, but other old targets (e.g. bcm47xx) are a different question).