Add support for ipxe load (nova, foc, fiasco)

This makes use of the iPXE sanboot command [1] which downloads and
boots an ISO image directly via HTTP. Therefore, your RUN_OPT needs
both

  --include image/iso   and
  --include load/ipxe

NOTE: The webserver serving the ISO image must support ranged requests,
      see [2].

[1] - http://ipxe.org/cmd/sanboot
[2] - http://forum.ipxe.org/showthread.php?tid=7295&pid=10482#pid10482

tool/run/boot_dir/foc

@@ -183,6 +183,12 @@ proc run_boot_dir_x86 {binaries} {
 
 		generate_tftp_config
 	}
+
+	if {[have_include "load/ipxe"]} {
+		create_ipxe_iso_config
+		update_ipxe_boot_dir
+		create_symlink_for_iso
+	}
 }