linuxManualConfig: restore functionality of isModular and buildDTBs

Since https://github.com/NixOS/nixpkgs/pull/221707 the `isModular`
and `buildDTBs` parameters no longer function properly -- they now
control only whether or not these elements are copied to the
outpath; they are build unconditionally in any case.

This commit reverts the part of #221707 which caused this behavior
change.

pkgs/os-specific/linux/kernel/manual-config.nix

@@ -232,7 +232,10 @@ stdenv.mkDerivation ({
     # replicated here to apply to older versions.
     # Makes __FILE__ relative to the build directory.
     "KCPPFLAGS=-fmacro-prefix-map=$(sourceRoot)/="
-  ] ++ extraMakeFlags;
+    kernelConf.target
+  ] ++ optional isModular "modules"
+    ++ optional buildDTBs "dtbs"
+    ++ extraMakeFlags;
 
   installFlags = [
     "INSTALL_PATH=$(out)"