extract kernel config options from base module

we then "import" them straight back into base.nix - it's not
as though you can opt out of having a kernel. But this means
they'll appear separately in the documentation

modules/base.nix

@@ -12,6 +12,9 @@ let
   type_service = pkgs.liminix.lib.types.service;
 
 in {
+  imports = [
+    ./kernel.nix                # this is a separate module for doc purposes
+  ];
   options = {
     # analogous to nixos systemPackages, but we don't symlink into
     # /run/current-system, we just add the paths in /etc/profile
@@ -28,23 +31,6 @@ in {
       default = "squashfs";
       type = types.str;
     };
-    kernel = {
-      src = mkOption { type = types.package; } ;
-      modular = mkOption {
-        type = types.bool;
-        default = true;
-        description = "support loadable kernel modules";
-      };
-      extraPatchPhase = mkOption {
-        default = "true";
-        type = types.lines;
-      } ;
-      config = mkOption {
-        # mostly the values are y n or m, but sometimes
-        # other strings are also used
-        type = types.attrsOf types.nonEmptyStr;
-      };
-    };
     boot = {
       commandLine = mkOption {
         type = types.listOf types.nonEmptyStr;
@@ -75,52 +61,6 @@ in {
         };
     };
 
-    kernel = rec {
-      modular = true; # disabling this is not yet supported
-      config = {
-        IKCONFIG = "y";
-        IKCONFIG_PROC = "y";
-        PROC_FS = "y";
-
-        KEXEC = "y";
-        MODULES = if modular then "y" else "n";
-        MODULE_SIG = if modular then "y" else "n";
-        DEBUG_FS = "y";
-
-        MIPS_BOOTLOADER_CMDLINE_REQUIRE_COOKIE = "y";
-        MIPS_BOOTLOADER_CMDLINE_COOKIE = "\"liminix\"";
-        MIPS_CMDLINE_DTB_EXTEND = "y";
-
-        # basic networking protocols
-        NET = "y";
-        UNIX = "y";
-        INET = "y";
-        IPV6 = "y";
-        PACKET = "y";           # for ppp, tcpdump ...
-        SYSVIPC= "y";
-
-        # disabling this option causes the kernel to use an "empty"
-        # initramfs instead: it has a /dev/console node and not much
-        # else.  Note that pid 1 is started *before* the root
-        # filesystem is mounted and it expects /dev/console to be
-        # present already
-        BLK_DEV_INITRD = lib.mkDefault "n"; # overriden by initramfs module
-
-        # s6-linux-init mounts this on /dev
-        DEVTMPFS = "y";
-        # some or all of these may be fix for "tmpfs: Unknown parameter 'mode'" error
-        TMPFS = "y";
-        TMPFS_POSIX_ACL = "y";
-        TMPFS_XATTR = "y";
-
-        FW_LOADER = "y";
-        FW_LOADER_COMPRESS = "y";
-        # We don't have a user helper, so we get multiple 60s pauses
-        # at boot time unless we disable trying to call it.
-        # https://lkml.org/lkml/2013/8/5/175
-        FW_LOADER_USER_HELPER = "n";
-      };
-    };
     boot.commandLine = [
       "console=ttyS0,115200 panic=10 oops=panic init=/bin/init loglevel=8"
       "root=${config.hardware.rootDevice}"

modules/kernel.nix

@@ -0,0 +1,86 @@
+## Kernel-related options
+## ======================
+
+
+{ lib, pkgs, config, ...}:
+let
+  inherit (lib) mkEnableOption mkOption types isDerivation hasAttr ;
+  inherit (pkgs.pseudofile) dir symlink;
+  inherit (pkgs.liminix.networking) address interface;
+  inherit (pkgs.liminix.services) bundle;
+
+  type_service = pkgs.liminix.lib.types.service;
+
+in {
+  options = {
+    kernel = {
+      src = mkOption { type = types.package; } ;
+      modular = mkOption {
+        type = types.bool;
+        default = true;
+        description = "support loadable kernel modules";
+      };
+      extraPatchPhase = mkOption {
+        default = "true";
+        type = types.lines;
+      } ;
+      config = mkOption {
+        description = ''
+          Kernel config options, as listed in Kconfig* files in the
+          kernel source tree. Do not include the leading "CONFIG_"
+          prefix when defining these.  Most values are "y", "n" or "m",
+          but sometimes other strings are also used.
+        '';
+        type = types.attrsOf types.nonEmptyStr;
+      };
+    };
+  };
+  config = {
+    kernel = rec {
+      modular = true; # disabling this is not yet supported
+      config = {
+        IKCONFIG = "y";
+        IKCONFIG_PROC = "y";
+        PROC_FS = "y";
+
+        KEXEC = "y";
+        MODULES = if modular then "y" else "n";
+        MODULE_SIG = if modular then "y" else "n";
+        DEBUG_FS = "y";
+
+        MIPS_BOOTLOADER_CMDLINE_REQUIRE_COOKIE = "y";
+        MIPS_BOOTLOADER_CMDLINE_COOKIE = "\"liminix\"";
+        MIPS_CMDLINE_DTB_EXTEND = "y";
+
+        # basic networking protocols
+        NET = "y";
+        UNIX = "y";
+        INET = "y";
+        IPV6 = "y";
+        PACKET = "y";           # for ppp, tcpdump ...
+        SYSVIPC= "y";
+
+        # disabling this option causes the kernel to use an "empty"
+        # initramfs instead: it has a /dev/console node and not much
+        # else.  Note that pid 1 is started *before* the root
+        # filesystem is mounted and it expects /dev/console to be
+        # present already
+        BLK_DEV_INITRD = lib.mkDefault "n"; # overriden by initramfs module
+
+        # s6-linux-init mounts this on /dev
+        DEVTMPFS = "y";
+        # some or all of these may be fix for "tmpfs: Unknown parameter 'mode'" error
+        TMPFS = "y";
+        TMPFS_POSIX_ACL = "y";
+        TMPFS_XATTR = "y";
+
+        FW_LOADER = "y";
+        FW_LOADER_COMPRESS = "y";
+        # We don't have a user helper, so we get multiple 60s pauses
+        # at boot time unless we disable trying to call it.
+        # https://lkml.org/lkml/2013/8/5/175
+        FW_LOADER_USER_HELPER = "n";
+      };
+    };
+  };
+}