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/watchdog.nix

@@ -0,0 +1,9 @@
+{ ... }:
+
+{
+  # The boards have a BMC watchdog controlled by IPMI
+  boot.kernelModules = [ "ipmi_watchdog" ];
+
+  # Enable systemd watchdog with 30 s interval
+  systemd.watchdog.runtimeTime = "30s";
+}