kernel-install: do not silently ignore files we can't read

kernel-install: do not silently ignore files we can't read

'test -r' is changed to 'test -f' everywhere. If the file exists but we
cannot read it, it would be better if we fail with a permission error. E.g. if
/etc/kernel/cmdline is unreadable, and we're running something as non-root, we
shouldn't fall back to /usr/lib/kernel/cmdline. This commit doesn't resolve
this fully, because we're not running with 'set -e', but this is a preparator
step.