Update image oat checksum when patching

Change-Id: I766d2745bc817a5cb3a36d62bc733bab43a31fe0
diff --git a/src/image.h b/src/image.h
index 399b143..852367d 100644
--- a/src/image.h
+++ b/src/image.h
@@ -71,6 +71,10 @@
     return oat_checksum_;
   }
 
+  void SetOatChecksum(uint32_t oat_checksum) {
+    oat_checksum_ = oat_checksum;
+  }
+
   byte* GetOatBegin() const {
     return reinterpret_cast<byte*>(oat_begin_);
   }