src/coreboot: put expose fmap in passthru

This commit exposes the `fmap` (flash chip partition table) used to
build the coreboot image as the `passthru.fmap` attribute so it can
be referenced from other expressions.

src/coreboot/default.nix

@@ -23,7 +23,7 @@ stdenv.mkDerivation {
   pname = "coreboot";
   inherit version;
 
-  passthru = { inherit src; };
+  passthru = { inherit src fmap; };
   inherit src;
 
   patches = let inherit (nixpkgsOnBuildForBuild) fetchpatch; in [