test_image_util.sh: introduce a board-specific hook

Introduces a board-specific hook, board_test_setup.

Each board overlay can implement it in its
scripts/board_specific_setup.sh.

BUG=b:62090123
TEST=With CL:*627264, the hook works for lakitu test image.

Change-Id: Id7622cdcfb54625c57e3d700fe1840b8cddba12f
Reviewed-on: https://chromium-review.googlesource.com/1062959
Commit-Ready: Edward Jee <edjee@google.com>
Tested-by: Edward Jee <edjee@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/build_library/test_image_util.sh b/build_library/test_image_util.sh
index e45a115..4eebb78 100755
--- a/build_library/test_image_util.sh
+++ b/build_library/test_image_util.sh
@@ -50,4 +50,8 @@
          ${image_name} --force_developer_mode
     fi
   fi
+
+  if type board_test_setup &>/dev/null; then
+    board_test_setup "${BUILD_DIR}/${image_name}"
+  fi
 }