We need the Broadcom firmware from the _linux-firmware_ package (and
nothing else). We also need _wpa_supplicant_, to configure the WiFi
connection. Finally, we need to configure _systemd-networkd_ to manage
the network device.
Need to add `rootwait` because the Raspberry Pi SD card reader is really
slow and does not initialize in time for the kernel to load the root
filesystem from it.
