When checking for CVE-2017-5715 (i.e. `check_CVE_2017_5715_linux()`),
if we can't inspect (with `readelf`) or decompress the Linux kernel
image, then we report there is no kernel image (i.e. `we need the
kernel image` or `kernel image missing`, respectively), which confuses
users when the associated file exists.
Instead use `kernel_err` to provide a correct and detailed description
of the problem (e.g. `missing '...' tool, please install it, usually
it's in the '...' package`), so the user can take the prescribed
action.
