buildUboot: add openssl to nativeBuildInputs

required by tools/kwbimage.c, tools/mxsimage.c and in various other
places too.
As those are tools running on the host, it's a nativeBuildInput.

pkgs/misc/uboot/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, fetchpatch, bc, dtc, python2
+{ stdenv, fetchurl, fetchpatch, bc, dtc, openssl, python2
 , hostPlatform
 }:
 
@@ -43,7 +43,7 @@ let
       patchShebangs tools
     '';
 
-    nativeBuildInputs = [ bc dtc python2 ];
+    nativeBuildInputs = [ bc dtc openssl python2 ];
 
     hardeningDisable = [ "all" ];