Fix warnings about unused variables
Fix warning about format string not being a string literal
repo: Use non-deprecated API with sufficiently new libarchive

Based on a patch Tobias Hunger <tobias.hunger@gmail.com>
admin: Error out if we see malformed or mismatched checksums for /boot

The kernel/initramfs both need to end in -SHA256 and match.
tests: Drop kernel modules from checksums

We can actually share these across different kernels.  Thanks to
Tobias Hunger <tobias.hunger@gmail.com> for pointing this out.