Split xeon specific configuration from base

To accomodate the raccoon knights workstation, some of the configuration
pulled by m/common/main.nix has to be removed. To solve it, the xeon
specific parts are placed into m/common/xeon.nix and only the common
configuration is at m/common/base.nix.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>

m/common/base/net.nix

@@ -0,0 +1,19 @@
+{ pkgs, ... }:
+
+{
+  networking = {
+    enableIPv6 = false;
+    useDHCP = false;
+
+    firewall = {
+      enable = true;
+      allowedTCPPorts = [ 22 ];
+    };
+
+    hosts = {
+      "84.88.53.236" = [ "ssfhead.bsc.es" "ssfhead" ];
+      "84.88.51.152" = [ "raccoon" ];
+      "84.88.51.142" = [ "raccoon-ipmi" ];
+    };
+  };
+}