[acpi] Support the "_RTXMAC_" format for ACPI-based MAC addresses

Some newer HP products expose the host-based MAC (HBMAC) address using
an ACPI method named "RTMA" returning a part-binary string of the form
"_RTXMAC_#<mac>#", where "<mac>" comprises the raw MAC address bytes.

Extend the existing support to handle this format alongside the older
"_AUXMAC_" format (which uses a base16-encoded MAC address).

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