build_image: provide USE flags when installing chromeos-dev

This fixes an issue where passing USE= to build_packages would always
cause build_image to fail.

BUG=none
TEST=create a USE-inconsistent package, run build_image, observe it working

Change-Id: Id9eb3891d9c292423c837a0d097a33155f03b794

R=davidjames@chromium.org,kliegs@chromium.org

Review URL: http://codereview.chromium.org/6677163
diff --git a/build_image b/build_image
index d9ce03e..ba9ac91 100755
--- a/build_image
+++ b/build_image
@@ -418,14 +418,14 @@
     root_dev_dir="${ROOT_FS_DIR}/usr/local"
 
   # Install developer packages described in chromeos-dev.
-  sudo INSTALL_MASK="${INSTALL_MASK}" ${EMERGE_BOARD_CMD} \
-      --root="${root_dev_dir}" --root-deps=rdeps \
+  sudo -E USE="${USE} ${EXTRA_USE}" INSTALL_MASK="${INSTALL_MASK}" \
+      ${EMERGE_BOARD_CMD} --root="${root_dev_dir}" --root-deps=rdeps \
       --usepkgonly -uDNv chromeos-dev ${EMERGE_JOBS}
 
   if [[ $FLAGS_preserve -eq ${FLAGS_TRUE} ]] ; then
     # Clean out unused packages
-    sudo INSTALL_MASK="${INSTALL_MASK}" ${EMERGE_BOARD_CMD} \
-      --root="${ROOT_FS_DIR}" --root-deps=rdeps \
+    sudo -E USE="${USE} ${EXTRA_USE}" INSTALL_MASK="${INSTALL_MASK}" \
+      ${EMERGE_BOARD_CMD} --root="${ROOT_FS_DIR}" --root-deps=rdeps \
       --usepkgonly --depclean ${EMERGE_JOBS}
   fi