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/rev.nix

@@ -0,0 +1,21 @@
+{ theFlake, ... }:
+
+let
+  # Prevent building a configuration without revision
+  rev = if theFlake ? rev then theFlake.rev
+    else throw ("Refusing to build from a dirty Git tree!");
+in {
+  # Save the commit of the config in /etc/configrev
+  environment.etc.configrev.text = rev + "\n";
+
+  # Keep a log with the config over time
+  system.activationScripts.configRevLog.text = ''
+    BOOTED=$(cat /run/booted-system/etc/configrev 2>/dev/null || echo unknown)
+    CURRENT=$(cat /run/current-system/etc/configrev 2>/dev/null || echo unknown)
+    NEXT=${rev}
+    DATENOW=$(date --iso-8601=seconds)
+    echo "$DATENOW booted=$BOOTED current=$CURRENT next=$NEXT" >> /var/configrev.log
+  '';
+
+  system.configurationRevision = rev;
+}