linux_rpi: Add some feature flags

Copied from linux_4_4 (except for the EFI stub thing).

Otherwise the firewall module fails to evaluate:
Failed assertions:
- This kernel does not support rpfilter

pkgs/os-specific/linux/kernel/linux-rpi.nix

@@ -16,6 +16,9 @@ stdenv.lib.overrideDerivation (import ./generic.nix (args // rec {
   };
 
   features.iwlwifi = true;
+  features.needsCifsUtils = true;
+  features.canDisableNetfilterConntrackHelpers = true;
+  features.netfilterRPFilter = true;
 
   extraMeta.hydraPlatforms = [];
 })) (oldAttrs: {