Support including DTB image

Bug: 111136242
Test: When BOARD_PREBUILT_DTB_DIR is set correctly,
generated $OUT/boot.img contains the DTB image.

Change-Id: I282e31b04cc60383377b9e9b54f8fe64a8140242
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index dcc083c..59a12f1 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -587,6 +587,11 @@
     cmd.append("--second")
     cmd.append(fn)
 
+  fn = os.path.join(sourcedir, "dtb")
+  if os.access(fn, os.F_OK):
+    cmd.append("--dtb")
+    cmd.append(fn)
+
   fn = os.path.join(sourcedir, "cmdline")
   if os.access(fn, os.F_OK):
     cmd.append("--cmdline")