make tftpboot work on devices with old u-boot

Some devices have a U-boot variant that does not accept a third
parameter on the "bootm" command, meaning we can't override the dtb
in the bootloader so have to smush it back into the kernel image

This doesn't work in QEMU but I think the problem is with the
U-Boot configuration for QEMU. It does work on at least one
hardware device so I'm pushing it anyway

Based on
https://gti.telent.net/raboof/liminix/src/branch/tftp-old-uboot

Co-authored-by:  Arnout Engelen <arnout@bzzt.net>

tests/tftpboot/test.nix

@@ -12,7 +12,7 @@ let derivation = (import liminix {
     img = derivation.outputs.tftpboot;
     uboot = derivation.outputs.u-boot;
     pkgsBuild = derivation.pkgs.pkgsBuildBuild;
-in pkgsBuild.runCommand "check" {
+in pkgsBuild.runCommand "check-${deviceName}" {
   nativeBuildInputs = with pkgsBuild; [
     expect
     socat
@@ -44,4 +44,11 @@ in {
   mipsLz = check  "qemu" {
     boot.tftp.compressRoot = true;
   };
+  # this works on real hardware but I haven't figured out how
+  # to make it work on qemu: it says
+  # "OF: fdt: No chosen node found, continuing without"
+
+  # mipsOldUboot = check  "qemu" {
+  #   boot.tftp.appendDTB = true;
+  # };
 }