After r302054 unloading an network interface driver on a kernel
without VIMAGE support would dereference a NULL point unconditionally
leading to a panic. Wrap the entire VIMAGE related code with #ifdefs
rather than just the decision making part to save an extra bit of
resources.

Reported by: np
Sponsored by: The FreeBSD Foundation
MFC After: 13 days
Approved by: re (marius)