OVMF: separate output for ovmf binaries

OVMF{,CODE,VARS}.fd are now available in a dedicated fd output, greatly
reducing the closure in the common case where only those files are used (a
few MBs versus several hundred MBs for the full OVMF).

Note: it's unclear why `dontPatchELF` is now necessary for the build to
pass (on my end, at any rate) but it doesn't make much sense to run this
fixup anyway,

Note: my reading of xen's INSTALL suggests that --with-system-ovmf should
point directly to the OVMF binary.  As such, the previous invocation was
incorrect (it pointed to the root of the OVMF tree).  In any case, I have
only built xen with `--with-system-ovmf`, I have not tested it.

Fixes https://github.com/NixOS/nixpkgs/issues/25854
Closes https://github.com/NixOS/nixpkgs/pull/25855

pkgs/applications/virtualization/OVMF/default.nix

@@ -15,6 +15,8 @@ in
 stdenv.mkDerivation (edk2.setup "OvmfPkg/OvmfPkg${targetArch}.dsc" {
   name = "OVMF-${version}";
 
+  outputs = [ "out" "fd" ];
+
   # TODO: properly include openssl for secureBoot
   buildInputs = [nasm iasl] ++ stdenv.lib.optionals (secureBoot == true) [ openssl ];
 
@@ -48,6 +50,13 @@ stdenv.mkDerivation (edk2.setup "OvmfPkg/OvmfPkg${targetArch}.dsc" {
       build -D CSM_ENABLE -D FD_SIZE_2MB ${if secureBoot then "-DSECURE_BOOT_ENABLE=TRUE" else ""}
     '';
 
+  postFixup = ''
+    mkdir -p $fd/FV
+    mv $out/FV/OVMF{,_CODE,_VARS}.fd $fd/FV
+  '';
+
+  dontPatchELF = true;
+
   meta = {
     description = "Sample UEFI firmware for QEMU and KVM";
     homepage = http://sourceforge.net/apps/mediawiki/tianocore/index.php?title=OVMF;