Feature: MAC lookup support for gPXE/iPXE

Adding support to lookup the gPXE/iPXE boot instructions via the MAC address of the machine. The main use case of this feature is to be able to use a generic gPXE/iPXE boot CD without having to control a DHCP server.

This can be used with [iPXE](http://ipxe.org) embedded scripts to boot machines via Cobbler if one has no control over the DHCP server and/or the next-server parameter:

    #!ipxe
    
    dhcp
    chain http://<your_cobbler_server>/cblr/svc/op/gpxe/mac/${net0/mac}