Comment 63 for bug 1624644

@jarnos #61
this is a separate issue fixed later, calculating upgradable packages leaves the upgradable packages in the cache, and u-u counts them as the reverse dependencies of the first kernel to be removed.
This keeps the first kernel on the system, but when there are no upgradable packages in a later run this kernel can be removed, too.

See:
https://github.com/mvo5/unattended-upgrades/commit/93d43fbcd53c5df5ce69a16b26a981bf06ce3085
https://github.com/mvo5/unattended-upgrades/commit/654898b05c933047ca8c97df655743aab0898db1
https://github.com/mvo5/unattended-upgrades/commit/1a39eb257ad786902de11add212879241919be44

In this particular case -extra was detected not autoremovable because of the dirty cache, but it was successfully removed when trying to remove linux-image-4.8.0-54-generic.
The printout is confusing indeed, but no harm was done, since only valid autoremovals were performed.