distinguish NFS versus TFTP boot by rootpath

Don't use DHCP 150 option to decide which protocol use to netboot. When
root-path includes ip address - go thru NFS, if ip address not exists in
root-path - go thru TFTP from server which ip address is in next-server. But
there is one limitation - only one tftp server in network to provide loader and
everything else. Does enybody use more than only one?

Submitted by: kczekirda
Sponsored by: Oktawave
MFC after: 3 weeks
Relnote: Yes
Differential Revision: https://reviews.freebsd.org/D8740