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

[hlef]

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 #50.

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