chromite: Add a stage to generate sysroot for the builds.

We want to start fuzzing testing for Chrome OS.
To do that we need a new stage to create a sysroot that can be
used by clusterfuzz to run the fuzz binaries.

BUG=chromium:807401

TEST=Chromite Unit tests pass.

Change-Id: I92d3e5dda0eac8b0ba15ea5f2e41cbfb4b131d83
Reviewed-on: https://chromium-review.googlesource.com/914937
Commit-Ready: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
diff --git a/scripts/cros_generate_sysroot_unittest.py b/scripts/cros_generate_sysroot_unittest.py
index df9a117..03a982c 100644
--- a/scripts/cros_generate_sysroot_unittest.py
+++ b/scripts/cros_generate_sysroot_unittest.py
@@ -83,6 +83,16 @@
     self.assertEquals(
         options.out_file, 'sysroot_chromeos-base_chromeos-chrome.tar.xz')
 
+  def testMultiplePkgsTargetName(self):
+    """Test getting the right target name with multiple pkgs."""
+    pkgs = "%s virtual/target-os" %constants.CHROME_CP
+    options = cros_gen.ParseCommandLine(
+        ['--board', BOARD, '--out-dir', self.tempdir,
+         '--package', pkgs])
+
+    self.assertEquals(
+        options.out_file, 'sysroot_chromeos-base_chromeos-chrome.tar.xz')
+
   def testExistingTarget(self):
     """Erroring out on pre-existing target."""
     options = self._Parse(['--out-file', TAR_NAME])