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>

doc/install.md

@@ -150,3 +150,13 @@ And update grub.
 ```
 # nix build .#nixosConfigurations.xeon02.config.system.build.kexecTree -v
 ```
+
+## Chain NixOS in same disk
+
+```
+menuentry 'NixOS' {
+        insmod chain
+        set root=(hd3,1)
+        configfile /boot/grub/grub.cfg
+}
+```