Mechanism to detect changed runtime images at boot

This patch adds a mechanism to detect if the current runtime image is
the same as during the last run.

This mechanism consist of several parts:
1. Reliable 'fingerprint' for an image
2. Safe way to store the image fingerprint
3. A helper to detect image changes and trigger a hook

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1101215
Change-Id: I120326e25de0a0de3e117204777891591b91c604
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>