buildLinux: take and propagate extraMakeFlags

This is just for practicity, as it allows users of buildLinux to pass
along extra flags they need in the kernel's make invocation. This makes,
for example, supporting LLVM _much_ easier, and could enable us in the
future to provide clang-built kernels.

pkgs/os-specific/linux/kernel/generic.nix

@@ -21,6 +21,9 @@
 , # Legacy overrides to the intermediate kernel config, as string
   extraConfig ? ""
 
+  # Additional make flags passed to kbuild
+, extraMakeFlags ? []
+
 , # kernel intermediate config overrides, as a set
  structuredExtraConfig ? {}
 
@@ -97,7 +100,7 @@ let
     in lib.concatStringsSep "\n" ([baseConfigStr] ++ configFromPatches);
 
   configfile = stdenv.mkDerivation {
-    inherit ignoreConfigErrors autoModules preferBuiltin kernelArch;
+    inherit ignoreConfigErrors autoModules preferBuiltin kernelArch extraMakeFlags;
     pname = "linux-config";
     inherit version;
 
@@ -116,7 +119,8 @@ let
     # e.g. "bzImage"
     kernelTarget = stdenv.hostPlatform.linux-kernel.target;
 
-    makeFlags = lib.optionals (stdenv.hostPlatform.linux-kernel ? makeFlags) stdenv.hostPlatform.linux-kernel.makeFlags;
+    makeFlags = lib.optionals (stdenv.hostPlatform.linux-kernel ? makeFlags) stdenv.hostPlatform.linux-kernel.makeFlags
+      ++ extraMakeFlags;
 
     prePatch = kernel.prePatch + ''
       # Patch kconfig to print "###" after every question so that
@@ -136,7 +140,8 @@ let
       export HOSTLD=$LD_FOR_BUILD
 
       # Get a basic config file for later refinement with $generateConfig.
-      make -C . O="$buildRoot" $kernelBaseConfig \
+      make $makeFlags \
+          -C . O="$buildRoot" $kernelBaseConfig \
           ARCH=$kernelArch \
           HOSTCC=$HOSTCC HOSTCXX=$HOSTCXX HOSTAR=$HOSTAR HOSTLD=$HOSTLD \
           CC=$CC OBJCOPY=$OBJCOPY OBJDUMP=$OBJDUMP READELF=$READELF \
@@ -176,7 +181,7 @@ let
   }; # end of configfile derivation
 
   kernel = (callPackage ./manual-config.nix {}) {
-    inherit version modDirVersion src kernelPatches randstructSeed lib stdenv extraMeta configfile;
+    inherit version modDirVersion src kernelPatches randstructSeed lib stdenv extraMakeFlags extraMeta configfile;
 
     config = { CONFIG_MODULES = "y"; CONFIG_FW_LOADER = "m"; };
   };

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

@@ -19,6 +19,8 @@ in {
   stdenv,
   # The kernel version
   version,
+  # Additional kernel make flags
+  extraMakeFlags ? [],
   # The version of the kernel module directory
   modDirVersion ? version,
   # The kernel source (tarball, git checkout, etc.)
@@ -173,7 +175,9 @@ let
         "KBUILD_BUILD_VERSION=1-NixOS"
         kernelConf.target
         "vmlinux"  # for "perf" and things like that
-      ] ++ optional isModular "modules";
+      ]
+      ++ optional isModular "modules"
+      ++ extraMakeFlags;
 
       installFlags = [
         "INSTALLKERNEL=${installkernel}"
@@ -325,7 +329,7 @@ stdenv.mkDerivation ((drvAttrs config stdenv.hostPlatform.linux-kernel kernelPat
     "ARCH=${stdenv.hostPlatform.linuxArch}"
   ] ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) [
     "CROSS_COMPILE=${stdenv.cc.targetPrefix}"
-  ];
+  ] ++ extraMakeFlags;
 
   karch = stdenv.hostPlatform.linuxArch;
 })