lib: Clean up how linux and gcc config is specified

Second attempt of 8929989614589ee3acd070a6409b2b9700c92d65; see that
commit for details.

This reverts commit 0bc275e634.

nixos/modules/system/boot/loader/raspberrypi/raspberrypi.nix

@@ -103,6 +103,6 @@ in
 
     system.build.installBootLoader = builder;
     system.boot.loader.id = "raspberrypi";
-    system.boot.loader.kernelFile = platform.kernelTarget;
+    system.boot.loader.kernelFile = linux-kernel.target;
   };
 }