[iscsi] Include IP address origin in iBFT

[iscsi] Include IP address origin in iBFT

The iBFT includes an "origin" field to indicate the source of the IP
address.  We use the heuristic of assuming that the source should be
"manual" if the IP address originates directly from the network device
settings block, and "DHCP" otherwise.  This is an imperfect guess, but
is likely to be correct in most common situations.

Originally-implemented-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>