[prefix] Add a generic raw image prefix

[prefix] Add a generic raw image prefix

Provide a generic raw image prefix, which assumes that the iPXE image
has been loaded in its entirety on a paragraph boundary.

The resulting .raw image can be loaded via RPL using an rpld.conf file
such as:

    HOST {
        ethernet = 00:00:00:00:00:00/6;
        FILE {
            path="ipxe.raw";
            load=0x2000;
        };
        execute=0x2000;
    };

Debugged-by: Johannes Heimansberg <git@jhe.dedyn.io>
Signed-off-by: Michael Brown <mcb30@ipxe.org>