U-Boot: Add platforms to legacy U-Boots

Or they will be tried to be built on x86_64 since #19990.

pkgs/misc/uboot/guruplug.nix

···
     mkdir -p $out/bin
     cp tools/{envcrc,mkimage} $out/bin
   '';
+
+  meta = {
+    platforms = [ "armv5tel-linux" ];
+  };
 }

pkgs/misc/uboot/nanonote.nix

···
     mkdir -p $out/bin
     cp tools/{envcrc,mkimage} $out/bin
   '';
+
+  meta = {
+    platforms = [ "mipsel-linux" ];
+  };
 }

pkgs/misc/uboot/sheevaplug.nix

···
     mkdir -p $out/bin
     cp tools/{envcrc,mkimage} $out/bin
   '';
+
+  meta = {
+    platforms = [ "armv5tel-linux" ];
+  };
 }