Bug#1054394: Postinst installs unsigned (unbootable) efi on secure boot systems

[sympathischerwal]

Hi,

I am running secure boot with my own keys.
I signed the efi binary myself with my own keys and put it
to the efi partition. On a systemd-boot upgrade, the postinst
overwrites these files, which made my bootable system unbootable.

Best,
Thomas