linux: only use uinstall when on armv7

The `uinstall` make target only exists for the armv7 architecture in
linux source, we should not use it if we are not building for armv7.

```
$ rg uinstall
arch/arm/Makefile
293:INSTALL_TARGETS     = zinstall uinstall install
327:  echo  '  uinstall      - Install U-Boot wrapped compressed kernel'
```

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

@@ -301,10 +301,10 @@ let
         export HOME=${installkernel}
       '';
 
-      # Some image types need special install targets (e.g. uImage is installed with make uinstall)
+      # Some image types need special install targets (e.g. uImage is installed with make uinstall on arm)
       installTargets = [
         (kernelConf.installTarget or (
-          /**/ if kernelConf.target == "uImage" then "uinstall"
+          /**/ if kernelConf.target == "uImage" && stdenv.hostPlatform.isArmv7 then "uinstall"
           else if kernelConf.target == "zImage" || kernelConf.target == "Image.gz" then "zinstall"
           else "install"))
       ];