linux: default stdenv.hostPlatform.linux-kernel

With this change, we can do a reasonable default build of Linux for
configurations that do not have a corresponding platforms.nix entry,
and where the user has not explicitly specified the linux-kernel
values.  This allows us to do best effort builds for obscure
architectures (I tested a build for s390x).

The platformName binding has not been used since
70cb705 ("Trying to make the linux kernels also cross-build."),
so I removed it.

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

@@ -47,15 +47,15 @@
   # symbolic name and `patch' is the actual patch.  The patch may
   # optionally be compressed with gzip or bzip2.
   kernelPatches ? []
-, ignoreConfigErrors ? stdenv.hostPlatform.linux-kernel.name != "pc"
+, ignoreConfigErrors ? stdenv.hostPlatform.linux-kernel.name or "" != "pc"
 , extraMeta ? {}
 
 , isZen      ? false
 , isLibre    ? false
 , isHardened ? false
 
 # easy overrides to stdenv.hostPlatform.linux-kernel members
-, autoModules ? stdenv.hostPlatform.linux-kernel.autoModules
+, autoModules ? stdenv.hostPlatform.linux-kernel.autoModules or true
 , preferBuiltin ? stdenv.hostPlatform.linux-kernel.preferBuiltin or false
 , kernelArch ? stdenv.hostPlatform.linuxArch
 , kernelTests ? []
@@ -128,11 +128,10 @@ let
       ++ lib.optionals (lib.versionAtLeast version "4.16") [ bison flex ]
       ++ lib.optional (lib.versionAtLeast version "5.2") pahole;
 
-    platformName = stdenv.hostPlatform.linux-kernel.name;
     # e.g. "defconfig"
-    kernelBaseConfig = if defconfig != null then defconfig else stdenv.hostPlatform.linux-kernel.baseConfig;
+    kernelBaseConfig = if defconfig != null then defconfig else stdenv.hostPlatform.linux-kernel.baseConfig or "defconfig";
     # e.g. "bzImage"
-    kernelTarget = stdenv.hostPlatform.linux-kernel.target;
+    kernelTarget = stdenv.hostPlatform.linux-kernel.target or "vmlinux";
 
     makeFlags = lib.optionals (stdenv.hostPlatform.linux-kernel ? makeFlags) stdenv.hostPlatform.linux-kernel.makeFlags
       ++ extraMakeFlags;

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

@@ -87,7 +87,8 @@ let
 
   isModular = config.isYes "MODULES";
 
-  kernelConf =  stdenv.hostPlatform.linux-kernel;
+  kernelConf = stdenv.hostPlatform.linux-kernel;
+  target = kernelConf.target or "vmlinux";
 
   buildDTBs = kernelConf.DTB or false;
 in
@@ -101,7 +102,7 @@ stdenv.mkDerivation ({
 
   depsBuildBuild = [ buildPackages.stdenv.cc ];
   nativeBuildInputs = [ perl bc nettools openssl rsync gmp libmpc mpfr zstd python3Minimal ]
-      ++ optional  (kernelConf.target == "uImage") buildPackages.ubootTools
+      ++ optional  (target == "uImage") buildPackages.ubootTools
       ++ optional  (lib.versionOlder version "5.8") libelf
       ++ optionals (lib.versionAtLeast version "4.16") [ bison flex ]
       ++ optionals (lib.versionAtLeast version "5.2")  [ cpio pahole zlib ]
@@ -297,8 +298,8 @@ stdenv.mkDerivation ({
   # Some image types need special install targets (e.g. uImage is installed with make uinstall)
   installTargets = [
     (kernelConf.installTarget or (
-      /**/ if kernelConf.target == "uImage" then "uinstall"
-      else if kernelConf.target == "zImage" || kernelConf.target == "Image.gz" then "zinstall"
+      /**/ if target == "uImage" then "uinstall"
+      else if target == "zImage" || target == "Image.gz" then "zinstall"
       else "install"))
   ];