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/fs.nix

@@ -0,0 +1,24 @@
+{ ... }:
+
+{
+  fileSystems."/" =
+    { device = "/dev/disk/by-label/nixos";
+      fsType = "ext4";
+    };
+
+  # Trim unused blocks weekly
+  services.fstrim.enable = true;
+
+  swapDevices =
+    [ { device = "/dev/disk/by-label/swap"; }
+    ];
+
+  # Tracing
+  fileSystems."/sys/kernel/tracing" = {
+    device = "none";
+    fsType = "tracefs";
+  };
+
+  # Mount a tmpfs into /tmp
+  boot.tmp.useTmpfs = true;
+}