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/ntp.nix

@@ -0,0 +1,9 @@
+{ pkgs, ... }:
+
+{
+  services.ntp.enable = true;
+
+  # Use the NTP server at BSC, as we don't have direct access
+  # to the outside world
+  networking.timeServers = [ "84.88.52.36" ];
+}