Put the device MAC address into a pre-shared read-only variable in the firewall

Currently the device MAC is generated and stored in the firewall, and accessible by other compartments through `firewall_mac_address_get`.

A downside of having to retrieve a copy through `firewall_mac_address_get` is that the resulting copy increases the state of compartments, and is yet another thing that must be reset if a compartment crashes ("reset critical"). See the discussion in https://github.com/CHERIoT-Platform/network-stack/pull/50.

Instead, it would be desirable to put the device MAC address into a pre-shared read-only variable in the firewall.