kexectools: Disable only on RISC-V if Linux.

The isKexecable flag treated Linux without kexec as just a normal
variant, when it really should be treated as a special case incurring
complexity debt to support.

lib/meta.nix

@@ -86,4 +86,6 @@ rec {
         then { system = elem; }
         else { parsed = elem; };
     in lib.matchAttrs pattern platform;
+
+  enableIfAvailable = p: if p.meta.available or true then [ p ] else [];
 }

lib/systems/inspect.nix

@@ -39,8 +39,6 @@ rec {
     isAndroid      = [ { abi = abis.android; } { abi = abis.androideabi; } ];
     isMusl         = with abis; map (a: { abi = a; }) [ musl musleabi musleabihf ];
 
-    isKexecable    = map (family: { kernel = kernels.linux; cpu.family = family; })
-                       [ "x86" "arm" "aarch64" "mips" ];
     isEfi          = map (family: { cpu.family = family; })
                        [ "x86" "arm" "aarch64" ];
     isSeccomputable = map (family: { kernel = kernels.linux; cpu.family = family; })

nixos/modules/system/boot/kexec.nix

@@ -1,7 +1,7 @@
 { config, pkgs, lib, ... }:
 
 {
-  config = lib.mkIf (pkgs.kexectools != null) {
+  config = lib.mkIf (pkgs.kexectools.meta.available) {
     environment.systemPackages = [ pkgs.kexectools ];
 
     systemd.services."prepare-kexec" =

pkgs/os-specific/linux/kexectools/default.nix

@@ -22,5 +22,6 @@ stdenv.mkDerivation rec {
     homepage = http://horms.net/projects/kexec/kexec-tools;
     description = "Tools related to the kexec Linux feature";
     platforms = platforms.linux;
+    badPlatforms = platforms.riscv;
   };
 }

pkgs/os-specific/linux/systemd/default.nix

@@ -41,7 +41,8 @@ in stdenv.mkDerivation rec {
   buildInputs =
     [ linuxHeaders libcap kmod xz pam acl
       /* cryptsetup */ libuuid glib libgcrypt libgpgerror libidn2
-      libmicrohttpd kexectools libseccomp libffi audit lz4 bzip2 libapparmor
+      libmicrohttpd ] ++ stdenv.lib.meta.enableIfAvailable kexectools ++
+    [ libseccomp libffi audit lz4 bzip2 libapparmor
       iptables gnu-efi
       # This is actually native, but we already pull it from buildPackages
       pythonLxmlEnv

pkgs/top-level/all-packages.nix

@@ -3131,9 +3131,7 @@ with pkgs;
 
   keepalived = callPackage ../tools/networking/keepalived { };
 
-  kexectools = if hostPlatform.isKexecable
-                 then callPackage ../os-specific/linux/kexectools { }
-               else null;
+  kexectools = callPackage ../os-specific/linux/kexectools { };
 
   keybase = callPackage ../tools/security/keybase { };