Fix initrd by adding missing generated postint scripts

Some maintainer scripts like linux-image-5.4.0-4-{amd,arm}64.postinst were missing, causing the initrd to not be created when installing the kernel image packages, which led to image build failures and unbootable images.

This happened because:

1. the debian/apertis/update-metadata script has been used to delete and then regenerated all the maintainer scripts
2. since the signed-code feature got disabled for amd64 and arm64 the respective binary packages lost the -unsigned suffix, for instance going from linux-image-5.4.0-4-amd64-unsigned.postinst to linux-image-5.4.0-4-amd64.postinst
3. due to the upstream .gitignore file disregarding /debian/ completely, changed and deleted files are reported in git status but new files are ignored, so the deletion of files like linux-image-5.4.0-4-amd64-unsigned.postinst got committed but the linux-image-5.4.0-4-amd64.postinst replacement got ignored by git

Previous releases where not affected because the refreshed version of debian/control.md5sum was not committed, so the metadata files got regenerated correctly by the --git-prebuild hook in the package builder pipeline: by committing the refreshed debian/control.md5sum the rpebuild action turned in a no-op and the missing files did not get generated.

To prevent this from happening again, drop /debian/ from .gitignore.