[acpi] Expose system MAC address via ${sysmac} setting

[acpi] Expose system MAC address via ${sysmac} setting

Expose the system MAC address (if any) via the ${sysmac} setting.
This allows scripts to access the system MAC address even when iPXE
has decided not to apply it to a network device (e.g. because the
cached DHCPACK MAC address was selected in order to match the
behaviour of a previous boot stage).

The setting is named ${sysmac} rather than ${acpimac} in order to
allow for forward compatibility with non-ACPI mechanisms that may
exist in future for specifying a system MAC address.

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