90kernel-modules: don't install any block driver if not needed

90kernel-modules: don't install any block driver if not needed

If hostonly mode is in use, and there isn't any block device included
as hostonly device, then the initramfs won't need any block driver.

Eg. for kdump built image, there could be only one nfs mount point,
or the initramfs may only execute some network operation then reboot.

In such case, skip block driver installation to save space and time.

Signed-off-by: Kairui Song <kasong@redhat.com>