Try to extract the RFC1048 data from PXE. If we get enough info we can skip
the bootp(). It removes unnecessary DHCP request from pxeloader.

Submitted by: kczekirda
Sponsored by: Oktawave
Initiated by: Matthew Dillon
Reviewed by: smh, gnn, bapt, oshogbo
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D9847