[netdevice] Add "chip" setting

Suggested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[netdevice] Reset MAC address when asked to clear the "mac" setting

Signed-off-by: Michael Brown <mcb30@ipxe.org>
Merge branch 'master' into osimages