lisa._kmod: Fix atomic headers sha1 fixup

FIX

CONFIG_IKHEADERS produces an archive of kernel headers that is preprocessed to some extent to reduce its size. This processing unfortunately breaks the sha1 checksum embedded in those headers by changing the content of the file without updating the sha1.

Kbuild then checks that checksum and rejects that header. We are therefore forced to fix the checksum embededd in the header.

So far, that fix was invasive in that it removed empty lines. That unfortunately results in a dirty kernel tree when that recipe is applied on a clean header. In that scenario (when CONFIG_IKHEADERS is disabled but the recipe applied), we end up with a modified header, leading to a "-dirty" uname, leading to a vermagic error when loading the module.

Fix that by making the checksum fixup less invasive.