nfs: do not panic on bootpc_init when no interfaces are found

Replaces panic with a warning message to allow kernel continue
when no bootp eligible network interfaces are found.

This avoids having to build a custom kernel when using a local root
file system on targets like powerpcspe that expects bootp/NFS by
default.

Reviewed by: rmacklem
MFC after: 2 weeks
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D34567

(cherry picked from commit 0b1b30d66450b20bf18fb4ab39bc6d4f5a6a28fe)