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/agenix.nix

@@ -0,0 +1,9 @@
+{ agenix, ... }:
+
+{
+  imports = [ agenix.nixosModules.default ];
+
+  environment.systemPackages = [
+    agenix.packages.x86_64-linux.default
+  ];
+}