[aoe] Allow AoE device to be described using an EFI device path

[aoe] Allow AoE device to be described using an EFI device path

There is no standard defined for AoE device paths in the UEFI
specification, and it seems unlikely that any standard will be adopted
in future.

Choose to construct an AoE device path using a concatenation of the
network device path and a SATA device path, treating the AoE major and
minor numbers as the HBA port number and port multiplier port number
respectively.

Signed-off-by: Michael Brown <mcb30@ipxe.org>