Function find_image 

pub fn find_image(
    kernel_dir: Option<&str>,
    release: Option<&str>,
) -> Option<PathBuf>

Find a bootable kernel image on the host.

kernel_dir: explicit kernel directory (e.g. from KTSTR_KERNEL). When set, only that directory is searched — no fallback to local build trees or host paths.

release: kernel release string (e.g. from uname -r). When None, falls back to reading /proc/sys/kernel/osrelease — the same value the kernel exposes via the uname(2) syscall, without the shell-out cost.

Without kernel_dir, searches local build trees (./linux, ../linux), /lib/modules/{release}/build, then host paths (/lib/modules/{release}/vmlinuz, /boot/vmlinuz-{release}, /boot/vmlinuz).