Introduce tri-state hostonly mode

Introduce tri-state hostonly mode

Add a new option --hostonly-mode which accept an <mode> parameter, so we have a tri-state hostonly mode:

    * generic: by passing "--no-hostonly" or not passing anything.
               "--hostonly-mode" has no effect in such case.
    * sloppy: by passing "--hostonly --hostonly-mode sloppy". This
              is also the default mode when only "--hostonly" is given.
    * strict: by passing "--hostonly --hostonly-mode strict".

Sloppy mode is the original hostonly mode, the new introduced strict
mode will allow modules to ignore more drivers or do some extra job to
save memory and disk space, while making the image less portable.

Also introduced a helper function "optional_hostonly" to make it
easier for modules to leverage new hostonly mode.

To force install modules only in sloppy hostonly mode, use the form:

hostonly="$(optional_hostonly)" instmods <modules>

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