rename flashable->flashimage, o.squashfs to rootfs

"rootfs" describes what it is for, "squashfs" merely says
how it's implemented

(also, rootfs-as-jffs2 will soon be added)

modules/flashimage.nix

@@ -0,0 +1,63 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+  inherit (lib) mkOption types concatStringsSep;
+  inherit (config.boot) tftp;
+in {
+  options = {
+  };
+  config = {
+    kernel = {
+      config = {
+        MTD_SPLIT_UIMAGE_FW = "y";
+      };
+    };
+
+    programs.busybox.applets = [
+      "flashcp"
+    ];
+
+    outputs.firmware =
+      let o = config.outputs; in
+      pkgs.runCommand "firmware" {} ''
+        dd if=${o.uimage} of=$out bs=128k conv=sync
+        dd if=${o.rootfs} of=$out bs=128k conv=sync,nocreat,notrunc oflag=append
+      '';
+    outputs.flashimage =
+      let o = config.outputs; in
+      pkgs.runCommand "flashimage" {} ''
+        mkdir $out
+        cd $out
+        ln -s ${o.firmware} firmware.bin
+        ln -s ${o.rootfs} rootfs
+        ln -s ${o.kernel} vmlinux
+        ln -s ${o.manifest} manifest
+        ln -s ${o.kernel.headers} build
+        ln -s ${o.uimage} uimage
+        ln -s ${o.dtb} dtb
+        ln -s ${o.flash-scr} flash.scr
+     '';
+
+    outputs.flash-scr =
+      let
+        inherit (pkgs.lib.trivial) toHexString;
+        inherit (config.hardware) flash;
+      in
+        pkgs.buildPackages.runCommand "" {} ''
+          imageSize=$(stat -L -c %s ${config.outputs.firmware})
+          cat > $out << EOF
+          setenv serverip ${tftp.serverip}
+          setenv ipaddr ${tftp.ipaddr}
+          tftp 0x$(printf %x ${tftp.loadAddress}) result/firmware.bin
+          erase 0x$(printf %x ${flash.address}) +\''${filesize})
+          cp.b 0x$(printf %x ${tftp.loadAddress}) 0x$(printf %x ${flash.address}) \''${filesize}
+          echo command line was ${builtins.toJSON (concatStringsSep " " config.boot.commandLine)}
+          EOF
+        '';
+
+  };
+}