linux_rpi: Specify defconfig in kernel expression

In particular, now the mainline kernel can be built on the RPi 1 as well
(so kernelBaseConfig should always be a mainline defconfig from now on).
And RPi 2 users can now use linux_rpi without doing the
`nixpkgs.config.platform = lib.systems.platforms.raspberrypi2;` dance.

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

@@ -15,6 +15,11 @@ stdenv.lib.overrideDerivation (buildLinux (args // rec {
     sha256 = "19lb1gxz21x1d5zdznzqfq60kxg7iqmyl6l0mb9qg2vrl8fcgnxk";
   };
 
+  defconfig = {
+    "armv6l-linux" = "bcmrpi_defconfig";
+    "armv7l-linux" = "bcm2709_defconfig";
+  }.${stdenv.system} or (throw "linux_rpi not supported on '${stdenv.system}'");
+
   features = {
     efiBootStub = false;
   } // (args.features or {});