cros_setup_toolchains: switch post cross packages to a virtual

Rather than maintain the list of packages to merge here, use the
new virtual.  This way we can do normal dep management there.

BUG=chromium:751852
TEST=precqs pass
CQ-DEPEND=CL:724108

Change-Id: I8785608157255f1d7b7a8b2978ffbe30acfec065
Reviewed-on: https://chromium-review.googlesource.com/724139
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index 27708dd..1ca4453 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -66,6 +66,7 @@
 # build), so we have to delay their installation.
 HOST_POST_CROSS_PACKAGES = (
     'dev-lang/rust',
+    'virtual/target-sdk-post-cross',
 )
 
 # New packages that we're in the process of adding to the SDK.  Since the SDK
@@ -73,6 +74,7 @@
 # so we have to list them here and wait.  Once it completes, entries here can
 # be removed so they'll end up on bots & dev's systems.
 NEW_PACKAGES = (
+    'virtual/target-sdk-post-cross',
 )
 
 # Enable the Go compiler for these targets.