rapsberrypi-eeprom: Include magic symlinks so command runs

The rpi-eeprom-update commands relies on a default
symlink. This is purely relative and points to critical, but is in fact
published by upstream.

Without this, the rpi-eeprom-update command fails with an error message
- and has no help options to make it easy to figure out the magic flags
  to get it to do the right thing.

pkgs/os-specific/linux/raspberrypi-eeprom/default.nix

@@ -28,6 +28,7 @@ stdenvNoCC.mkDerivation rec {
 
     cp rpi-eeprom-config rpi-eeprom-update $out/bin
     cp -r firmware/{beta,critical,old,stable} $out/share/rpi-eeprom
+    cp -P firmware/default firmware/latest $out/share/rpi-eeprom
   '';
 
   fixupPhase = ''