src/image: fix ability to boot from NORMAL region

commit fa3da49c84 "coreboot: update
patch to add master header in both images" still isn't causing
coreboot to put a master header into the NORMAL region.  As a
result, images built after that commit but before this one will
always use the FALLBACK region.

This commit adds the master header explicitly, since I still can't
seem to get coreboot to do that on its own.

src/image/default.nix

@@ -43,8 +43,17 @@ nixpkgsOnBuildForHost.stdenv.mkDerivation {
       runHook preBuild
       cp ${coreboot}/coreboot.rom .
       chmod +w coreboot.rom
+
+      ${update-cbfs "FALLBACK"}
+      cbfstool coreboot.rom print -rFALLBACK
+
+      # unfortunately this is the only way to trick cbfstool into
+      # putting a "header pointer" in both regions:
+      cbfstool coreboot.rom read     -r FALLBACK -f half.img
+      cbfstool coreboot.rom write -F -r NORMAL   -f half.img
+      rm half.img
       cbfstool coreboot.rom print -rNORMAL
-      ${lib.concatMapStrings (region: update-cbfs region) [ "NORMAL" "FALLBACK" ]}
+
       runHook postBuild
     '';