linux: Compress kernel modules

This reduces the kernel package from 185 to 62 MiB, for a neglible
boot time cost.

pkgs/os-specific/linux/kernel/common-config.nix

@@ -21,6 +21,12 @@
 with stdenv.lib;
 
 ''
+  # Compress kernel modules for a sizable disk space savings.
+  ${optionalString (versionAtLeast version "3.18") ''
+    MODULE_COMPRESS y
+    MODULE_COMPRESS_XZ y
+  ''}
+
   # Debugging.
   DEBUG_KERNEL y
   TIMER_STATS y

pkgs/os-specific/linux/kmod/default.nix

@@ -8,11 +8,9 @@ stdenv.mkDerivation rec {
     sha256 = "10lzfkmnpq6a43a3gkx7x633njh216w0bjwz31rv8a1jlgg1sfxs";
   };
 
-  # Disable xz/zlib support to prevent needing them in the initrd.
+  buildInputs = [ pkgconfig libxslt xz /* zlib */ ];
 
-  buildInputs = [ pkgconfig libxslt /* xz zlib */ ];
-
-  configureFlags = [ "--sysconfdir=/etc" /* "--with-xz" "--with-zlib" */ ];
+  configureFlags = [ "--sysconfdir=/etc" "--with-xz" /* "--with-zlib" */ ];
 
   patches = [ ./module-dir.patch ];