virtualbox: Allow building with extension pack.

The extension pack is needed in order to have USB 2.0, RDP and PXE boot support
and is _not_ part of the free version, so please read their conditions before
using it:

https://www.virtualbox.org/wiki/VirtualBox_PUEL

Further information and details about the extension pack is available here:

https://www.virtualbox.org/manual/ch01.html#intro-installing

I'm wimping out here and use requireFile to ask the user to fetch the extension
pack manually, simply because I'm not a lawyer. I've seen other distributions
that fetch the extension packs directly from upstream, but I'm not sure if that
is according to Oracle's PUEL.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/applications/virtualization/virtualbox/default.nix

@@ -4,6 +4,7 @@
 , xorriso, makeself, perl, pkgconfig
 , javaBindings ? false, jdk ? null
 , pythonBindings ? false, python ? null
+, enableExtensionPack ? false, requireFile ? null, patchelf ? null
 }:
 
 with stdenv.lib;
@@ -11,6 +12,7 @@ with stdenv.lib;
 let
 
   version = "4.2.4";
+  extpackRevision = "81684";
 
   forEachModule = action: ''
     for mod in \
@@ -29,6 +31,14 @@ let
     done
   '';
 
+  extensionPack = requireFile {
+    name = "Oracle_VM_VirtualBox_Extension_Pack-${version}-${extpackRevision}"
+         + ".vbox-extpack";
+    # Has to be base16 because it's used as an input to VBoxExtPackHelperApp!
+    sha256 = "62078e057a4ab56aec5ac086746522b3d94787333d0444169471fa5152c609ed";
+    url = "https://www.virtualbox.org/wiki/Downloads";
+  };
+
 in stdenv.mkDerivation {
   name = "virtualbox-${version}-${kernel.version}";
 
@@ -108,6 +118,15 @@ in stdenv.mkDerivation {
         ln -s "$libexec/$file" $out/bin/$file
     done
 
+    ${optionalString enableExtensionPack ''
+      "$libexec/VBoxExtPackHelperApp" install \
+        --base-dir "$libexec/ExtensionPacks" \
+        --cert-dir "$libexec/ExtPackCertificates" \
+        --name "Oracle VM VirtualBox Extension Pack" \
+        --tarball "${extensionPack}"
+        --sha-256 "${extensionPack.outputHash}"
+    ''}
+
     # Create and fix desktop item
     mkdir -p $out/share/applications
     sed -i -e "s|Icon=VBox|Icon=$libexec/VBox.png|" $libexec/virtualbox.desktop