kernel-install: make version/kernel image parameters optional for "add"

kernel-install: make version/kernel image parameters optional for "add"

Let's make kernel-install a bit easier to use:

If the kernel version is not specified, let's imply "uname -r", so that
we regnerate the entries for the current kernel.

If the kernel image is not specified let's imply using
/usr/lib/modules/$version/vmlinuz, i.e. the location distros like Fedora
drop the kernel into, which we generally recommend people to use.

If the kernel is not found there, don't try to automatically pick the
kernel path, and fail, as before.