Make sync_build_test and build_autotest support workon.

build_autotest.sh

 #!/bin/bash
 
 # Copyright (c) 2009 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.
 
 # This script makes autotest client tests inside a chroot environment.  The idea
 # is to compile any platform-dependent autotest client tests in the build
 # environment, since client systems under test lack the proper toolchain.
 #
 # The user can later run autotest against an ssh enabled test client system, or
 # install the compiled client tests directly onto the rootfs image.
 
 . "$(dirname "$0")/common.sh"
 
 get_default_board
 
-./autotest --build=all --board="${DEFAULT_BOARD}" $@
+DEFINE_string board "$DEFAULT_BOARD" \
+    "The board for which you are building autotest"
 
+FLAGS "$@" || exit 1
+
+if [[ -n "${CROS_WORKON_SRCROOT}" ]]; then
+  if [[ -z "${FLAGS_board}" ]]; then
+    setup_board_warning
+    exit 1
+  fi
+  emerge-${FLAGS_board} autotest-tests
+else
+  ./autotest --noprompt --build=all --board="${DEFAULT_BOARD}" $@
+fi
+