This implements the same behavior as it is used in the autoupdater [1].

This is for example required to allow the manual installation of
firmware upgrades via the config mode on devices which where migrated
from swconfig to DSA. Otherwise the image will always be invalid.

[1] https://github.com/freifunk-gluon/packages/commit/b80428166454d184f356e5a14a144911d9ee11aa