clean old chromeos-base/chromeos-bsp-null

We are converting virtual/chromeos-bsp to a new style virtual, but we
have to clean out the old one first as it has a PROVIDE= which messes
up the upgrade process.

BUG=chromium-os:38307
TEST=`./update_chroot` uninstalled this pkg for all generic boards
CQ-DEPEND=CL:42188

Change-Id: Ie23bf462a178bf1085a4700155b779e9651ad24d
Reviewed-on: https://gerrit.chromium.org/gerrit/42528
Reviewed-by: Anush Elangovan <anush@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>

chroot_version_hooks.d/51_new_style_virtual_chromeos-bsp

@@ -0,0 +1,33 @@
+# Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Upgrade people to the new style generic virtual/chromeos-bsp.
+
+opkg="chromeos-base/chromeos-bsp-null"
+
+export CLEAN_DELAY=0
+
+update() {
+  local root=$1 board=$2 emerge
+
+  if [[ -z ${board} ]]; then
+    board="root"
+    emerge="sudo -E emerge"
+  else
+    emerge="emerge-${board}"
+  fi
+
+  if portageq has_version ${root} ${opkg}; then
+    ${emerge} -Cq ${opkg}
+  fi
+}
+
+update / &
+
+for board_root in /build/*; do
+  board=${board_root##*/}
+  update ${board_root} ${board} &
+done
+
+wait