Use DHCP for first stage boot

This approach was recommended by nixos.wiki, and formerly by
wiki.nixos.org, which has since switched to instead mentioning
`boot.initrd.network.udhcpc.enable = true;`. It's not entirely clear to
me which has an advantage over the other.

This kernel parameter means that the kernel itself does the DHCP
request, which is pretty neat! That seems to get set up in this file,
though I haven't tracked down the exact details:

https://github.com/torvalds/linux/blob/master/net/ipv4/ipconfig.c

It seems like udhcpc may _also_ be enabled (since networking.useDHCP is
set for the main system?), so maybe that could be disabled, but this
appears to work so I'm rolling with it for the time being!

Without the kernel param set, udhcpc tries and fails with errors like
this:

```
<<< NixOS Stage 1 >>>

loading module dm_mod...
loading module af_packet...
running udev...
Starting systemd-udevd version 255.9
bringing up network interface eno1...
acquiring IP address via DHCP on eno1...
udhcpc: started, v1.36.1
udhcpc: broadcasting discover
udhcpc: broadcasting discover
udhcpc: broadcasting discover
udhcpc: no lease, failing
Passphrase for /dev/disk/by-uuid/00000000-0000-0000-0000-000000000000: _
```

Compare with a working output:

```
<<< NixOS Stage 1 >>>

loading module dm_mod...
loading module af_packet...
running udev...
Starting systemd-udevd version 255.9
bringing up network interface eno1...
acquiring IP address via DHCP on eno1...
udhcpc: started, v1.36.1
udhcpc: broadcasting discover
udhcpc: broadcasting discover
udhcpc: broadcasting discover
udhcpc: broadcasting select for 192.168.1.20, server 192.168.1.1
udhcpc: lease of 192.168.1.20 obtained from 192.168.1.1, lease time 43200
Passphrase for /dev/disk/by-uuid/00000000-0000-0000-0000-000000000000: _
```

bert/configuration.nix

@@ -14,6 +14,7 @@
   boot.loader.efi.canTouchEfiVariables = true;
 
   # Set up SSH unlocking
+  boot.kernelParams = [ "ip=dhcp" ];
   boot.initrd = {
     availableKernelModules = [ "e1000e" ];
     network = {