Verify that SCRIPTS_DIR exists in build_image.

This hack is necessary for old chroots to ensure that we source a good
common.sh when the one inside the chroot is out of date. We'll be able
to get rid of this after everybody has updated their chroot to have a working
version of common.sh and/or we have deprecated running scripts from the
source directory.

Also fix logic for when common.sh is missing to exit properly.

BUG=chromium-os:11598
TEST=Verify that build_image still works when the old common.sh is present in
     the chroot.

Change-Id: I2e8c2fd5ad06446e669ad665ee2e70f4364958fe

Review URL: http://codereview.chromium.org/6286084

build_image

@@ -9,17 +9,17 @@
 # the given target's root with binary packages turned on. This script will
 # build the Chrome OS image using only pre-built binary packages.
 
-# --- BEGIN COMMON.SH BOILERPLATE ---
 # Load common CrOS utilities.  Inside the chroot this file is installed in
 # /usr/lib/crosutils.  Outside the chroot we find it relative to the script's
 # location.
 find_common_sh() {
-  local common_paths=(/usr/lib/crosutils $(dirname "$(readlink -f "$0")"))
+  local common_paths=(/usr/lib/crosutils $(dirname "$0"))
   local path
 
   SCRIPT_ROOT=
   for path in "${common_paths[@]}"; do
-    if [ -r "${path}/common.sh" ]; then
+    local common="${path}/common.sh"
+    if ([ -r "${common}" ] && . "${common}" && [ -d "${SCRIPTS_DIR}" ]); then
       SCRIPT_ROOT=${path}
       break
     fi
@@ -27,8 +27,7 @@ find_common_sh() {
 }
 
 find_common_sh
-. "${SCRIPT_ROOT}/common.sh" || (echo "Unable to load common.sh" && exit 1)
-# --- END COMMON.SH BOILERPLATE ---
+. "${SCRIPT_ROOT}/common.sh" || ! echo "Unable to load common.sh" || exit 1
 
 # Load functions and constants for chromeos-install
 [ -f /usr/lib/installer/chromeos-common.sh ] && \