rkbin: Update license to unfreeRedistributableFirmware

rkbin packages the firmware required for initialisation of rockchip
SoC's.  There is a minor difference between unfreeRedistributable and
unfreeRedistributableFirmware, in that nixpkgs considers the firmware
variant "free" to allow it to be used to build images and channel
releases.

This change will allow uboot to be built and cached for rockchip devices.

pkgs/by-name/rk/rkbin/package.nix

@@ -29,7 +29,7 @@ stdenv.mkDerivation {
   meta = with lib; {
     description = "Rockchip proprietary bootloader blobs";
     homepage = "https://github.com/rockchip-linux/rkbin";
-    license = licenses.unfreeRedistributable;
+    license = licenses.unfreeRedistributableFirmware;
     maintainers = with maintainers; [ thefossguy ];
     platforms = lib.platforms.all;
   };