linux: inline "commonMakeFlags"

Since 7a9b6ac39a ("kernel: Enable cross compiling"), commonMakeFlags
has only been used once.  Since these flags aren't "common" between
anything any more, they should just be inlined to make the expression
easier to read.

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

@@ -68,11 +68,6 @@ let
     cp -av $3 $4
   ''; };
 
-  commonMakeFlags = [
-    "O=$(buildRoot)"
-  ] ++ lib.optionals (stdenv.hostPlatform.linux-kernel ? makeFlags)
-    stdenv.hostPlatform.linux-kernel.makeFlags;
-
   drvAttrs = config_: kernelConf: kernelPatches: configfile:
     let
       config = let attrName = attr: "CONFIG_" + attr; in {
@@ -335,13 +330,15 @@ stdenv.mkDerivation ((drvAttrs config stdenv.hostPlatform.linux-kernel kernelPat
   hardeningDisable = [ "bindnow" "format" "fortify" "stackprotector" "pic" "pie" ];
 
   # Absolute paths for compilers avoid any PATH-clobbering issues.
-  makeFlags = commonMakeFlags ++ [
+  makeFlags = [
+    "O=$(buildRoot)"
     "CC=${stdenv.cc}/bin/${stdenv.cc.targetPrefix}cc"
     "HOSTCC=${buildPackages.stdenv.cc}/bin/${buildPackages.stdenv.cc.targetPrefix}cc"
     "ARCH=${stdenv.hostPlatform.linuxArch}"
   ] ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) [
     "CROSS_COMPILE=${stdenv.cc.targetPrefix}"
-  ] ++ extraMakeFlags;
+  ] ++ (stdenv.hostPlatform.linux-kernel.makeFlags or [])
+    ++ extraMakeFlags;
 
   karch = stdenv.hostPlatform.linuxArch;
 } // (optionalAttrs (pos != null) { inherit pos; }))