cross-canary: Add simple test to validate cross work

At least, to a basic level.

overlay/mobile-nixos/cross-canary/test.nix

@@ -0,0 +1,45 @@
+{ stdenv, runCommandNoCC, runtimeShell, busybox, hello-mruby, pkgsBuildBuild }:
+
+let
+  inherit (stdenv) system;
+  emulators = {
+    "aarch64-linux" = "qemu-aarch64";
+    "armv7l-linux" = "qemu-arm";
+  };
+  emulator =
+    if stdenv.buildPlatform == stdenv.hostPlatform then ""
+    else "${pkgsBuildBuild.qemu}/bin/${emulators.${system}}"
+  ;
+  mkTest = what: script: runCommandNoCC "cross-canary-${what}-${stdenv.system}" {} ''
+    (
+    PS4=" $ "
+    set -x
+
+    ${script}
+
+    )
+
+    # Everything went okay, mark the build as a success!
+    touch $out
+  '';
+in
+
+# We're not creating a "useless" canary when there is no cross compilation.
+if stdenv.buildPlatform == stdenv.hostPlatform then {} else
+{
+  busybox = mkTest "busybox" ''
+    # Checks that busybox works
+    ${emulator} ${busybox}/bin/busybox uname -a
+    ${emulator} ${busybox}/bin/busybox sh -c 'echo busybox works...'
+  '';
+
+  mruby = mkTest "mruby" ''
+    # Checks that mruby works at its most basic.
+    ${emulator} ${hello-mruby}/bin/hello
+  '';
+
+  runtimeShell = mkTest "runtimeShell" ''
+    # And what about runtimeShell?
+    ${emulator} ${runtimeShell} -c 'echo runtimeShell works...'
+  '';
+}

overlay/overlay.nix

@@ -136,6 +136,8 @@ in
       make-flashable-zip = callPackage ./mobile-nixos/android-flashable-zip/make-flashable-zip.nix {};
 
       map-dtbs = callPackage ./mobile-nixos/map-dtbs {};
+
+      cross-canary-test = callPackage ./mobile-nixos/cross-canary/test.nix {};
     };
 
     imageBuilder = callPackage ../lib/image-builder {};