"ubimage" module contains ubifs image + instructions

Presently either you run this from U-Boot or you figure out for
yourself how to kexecboot into a recovery system :-)

modules/ubifs.nix

@@ -35,9 +35,10 @@ in
         in runCommand "mkfs.ubifs" {
           depsBuildBuild = [ mtdutils ];
         } ''
-          mkdir -p $TMPDIR/empty/nix/store/ $TMPDIR/empty/secrets
+          mkdir -p $TMPDIR/empty/nix/store/ $TMPDIR/empty/secrets $TMPDIR/empty/boot
           cp ${systemConfiguration}/bin/activate $TMPDIR/empty/activate
           ln -s ${pkgs.s6-init-bin}/bin/init $TMPDIR/empty/init
+          cp ${config.system.outputs.uimage} $TMPDIR/empty/boot/uimage
           mkdir -p $TMPDIR/empty/nix/store
           for path in $(cat ${systemConfiguration}/etc/nix-store-paths) ; do
             (cd $TMPDIR/empty && cp -a $path .$path)

modules/ubimage.nix

@@ -0,0 +1,112 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+  inherit (lib) mkIf mkEnableOption mkOption types concatStringsSep;
+  loadaddr = config.boot.tftp.loadAddress;
+  cfg = config.boot.tftp;
+  instructions = pkgs.writeText "INSTALL.md" ''
+
+# First-time installation
+
+First-time installation of a UBIFS Liminix system presently can only
+be done from the U-Boot command line (or from a ramdisk-based recovery
+system: see "kexecboot" but we don't have detailed instructions for
+this process yet).
+
+These steps were tested on a Belkin RT3200 (also known as Linksys
+E8450).  Other devices may be set up differently, so use them as
+inspiration and don't just paste them blindly. Consult the Liminix
+manual for how to connect a serial cable to your device and get into
+U-Boot
+
+1) determine which MTD device is being used for UBI and the partition name:
+
+    uboot>  ubi part
+    Device 0: ubi0, MTD partition ubi
+
+In this case the important value is `ubi0`
+
+2) list the available volumes and create a new one on which to install Liminix
+
+    uboot> ubi info l
+    [ copious output scrolls past ]
+
+Expect there to be existing volumes and for some or all of them to be
+important. Unless you know what you're doing, don't remove anything
+whose name suggests it's related to uboot, or any kind of backup or
+recovery partition. To see how much space is free:
+
+    uboot> ubi info
+    [ ... ]
+    UBI: available PEBs:             823
+
+Now we can make our new root volume
+
+    uboot> ubi create liminix -
+
+3) transfer the root filesystem from the build system and write it
+to the new volume
+
+    uboot> setenv serverip ${cfg.serverip}
+    uboot> setenv ipaddr ${cfg.ipaddr}
+    uboot> tftpboot ${loadaddr} result/rootfs
+    uboot> ubi write ${loadaddr} liminix $filesize
+
+Now we have the root filesystem installed on the device. You
+can even mount it and poke around using `ubifsmount ubi0:liminix;
+ubifsls  /`
+
+4) optional: before you configure the device to boot into Liminix
+automatically, you can try booting it by hand to see if it works:
+
+    uboot> ubifsmount ubi0:liminix
+    uboot> ubifsload ${loadaddr} boot/uimage
+    uboot> bootm ${loadaddr}
+
+Once you've done this and you're happy with it, reset the device to
+U-Boot. You don't need to recreate the volume but you do need to
+repeat step 3.
+
+5) Instructions for configuring autoboot are likely to be very
+device-dependent. On the Linksys E8450/Belkin RT3200, the environment
+variable `boot_production` governs what happens on a normal boot, so
+you could do
+
+    uboot> setenv boot_production 'led $bootled_pwr on ; ubifsmount ubi0:liminix; ubifsload ${loadaddr} boot/uimage; bootm  ${loadaddr}'
+
+On other devices, some detective work may be needed. Try running
+`printenv` and look for likely commands, try looking at the existing
+boot process, maybe even try looking for documentation for that device.
+
+6) Now you can reboot the device into Liminix
+
+    uboot> reset
+
+'';
+in {
+  options.system.outputs = {
+    ubimage = mkOption {
+      type = types.package;
+      description = ''
+        UBIFS image and instructions to install it
+      '';
+    };
+  };
+
+  config = mkIf (config.rootfsType == "ubifs") {
+    system.outputs = {
+      ubimage =
+        let o = config.system.outputs; in
+        pkgs.runCommand "ubimage" {} ''
+          mkdir $out
+          cd $out
+          ln -s ${o.rootfs} rootfs
+          ln -s ${instructions} INSTALL
+       '';
+    };
+  };
+}